"60% of Kafi is Weak"
Al Kafi is the gateway to the teachings of Ahlul Bayt and a source of guidance for the believers. All of the most crucial components of our aqida and spirituality are summed up in Al Kafi. Sayyed Khomeini famously once said, commenting on the necessity to consult Al Kafi:
"Do you think it is enough [kafi] for our religious life to have its laws summed up in al-Kafi and then placed upon a shelf?"
Indeed, every Shia who wants to take his religious learning seriously should consult the book of Al Kafi and learn from its hadiths.
Despite how important Al Kafi is, many Shia laymen are afraid of reading it! An important question a lot of laymen have is whether they can read Al Kafi without fear of being misguided.
Unfortunately, because of modern mystic trends, people have gotten the impression that only a scholar can read hadith from books like Al Kafi and benefit from them, and that if a layman delves into hadith he will fall into misguidance and error. This impression is seriously mistaken. Though it is not the focus of our discussion, we would still like to call our attention to a few important lectures which dispel this misconception and which can help a reader navigate through Shia hadith on a website like thaqalayn.net: "How Does a Non-Specialist Deal With the Quran and Sunnah" by Sayyed Ali Abul Hasan (Part 1) (Part 2); and "An English Speaker's Guide to Shia Hadith." A post will be written later summarizing these points in more detail.
Anyways, one of the many reasons for why Shia laymen are afraid of reading Shia hadith is because 60% of the narrations in Al Kafi have a weak chain of narrators:
According to the great Imami scholar Zayn al-Din al-`Amili, known as al-Shahid al-Thani (911-966/1505-1559), who examined the asnad or the chains of transmission of al-Kafi's traditions, [Al-Kafi] consists of 5072 sahih, 144 hasan, 1118 muwathaq, 302 qawi and 9485 traditions which are categorized as daif. (Al-Islam)
We should not try to shy away from this fact or come up with elaborate gymnastics around it. Rather, with the right understanding of how to read Al Kafi, this fact should not hinder a reader from properly learning from the book of Al Kafi.
The "Roman Concrete" of Hadith Books 
Like "Roman Concrete," the messages and contents of Al Kafi have a sort of self-corroborating property to them. The result of Kulayni's masterful collection is that even if one were to remove all of the inauthentic hadith in Al Kafi, and only study the authentic narrations one could still walk away from the book with an excellent understanding of Tashayyu. So even though 60% of the narrations have weak chains, this fact will not compromise the integrity of the book and the core Shia beliefs it stands for.
Sunnis jump to point out that 60% of the narrations in Al Kafi are weak in chain. But this does not discredit Shia Hadith in any way. A book of hadith is not magically tainted because it decides to quote weak narrations. And conversely, if one were to remove those 60% of hadith which are weak, and leave only the authentic narrations, Al Kafi would not magically grow wings and begin speaking to us and calling down angels. A book of hadith is not special because it chooses to only quote authentic narrations.
Bukhari contains only around 2600 narrations without repetitions. Muslim contains around 4000 without repetition. If Al Kafi contains over 6000 authentic narrations, then on what two legs can a Sunni say that Al Kafi does not pass muster to be a serious book of hadith?
The real question a Sunni should ask is whether if one were to remove all of the weak hadith, would we suffer some sort of irreparable loss of core Shia beliefs, and would Al Kafi’s integrity and internal consistency would be compromised. But as mentioned above, we say almost all of the chapters in Al Kafi are either anchored around an authentic hadith from which the rest of the hadith can be derived, or they have an authentic supporting hadith in another book. So even if one were to ignore the weak hadiths in the chapters, they would not lose anything of the contents.
If we want to understand Al Kafi properly, we should turn our attention to the matn of the hadiths in the book rather than strictly on the chains. A hadith consists of two parts: the isnad, and the matn. The isnad is the chain of narrators: the list of people a hadith went through to reach us. The matn is the contents of a hadith: what the Prophet or Imams are alleged to have said.
The isnad and the matn are two independent considerations when determining the authenticity of a hadith. A hadith can have a weak isnad (chain of narrators) but its contents can be strong and corroborated by other hadith or through other evidences. And conversely—although this happens extremely rarely—a hadith can have a strong isnad, but its contents can be questionable.
As an example of a hadith with a weak chain but reliable content consider the following hadith in Al Kafi which says: "All water is clean until it is found out that it is not clean.” Both chains Kulayni narrates this from are weak. However, the actual message of the hadith is well-known and reliably attested principle in Shia fiqh. In Qurb al Isnad, from an authentic chain it is narrated that: "Everything is pure until you learn it is impure. If you come to know, then it is dirty. And if you do not know, then there is nothing upon you." And even if, for the sake of argument, we decided to toss away this hadith from Qurb al Isnad, we could still derive the idea that "all water is clean until it is found out that it is not clean" from first principles through other hadith. Another authentic hadith says: "the messenger of Allah صلى الله عليه واله said: my Umma are absolved [not held liable] of nine things:... what they do not know..." (Al Khisal). So with a proper understanding and a strong foundation in authentic narrations, one can easily corroborate the contents of this chapter of Kafi.
Another example of a hadith with weak chains in Kafi but whose contents are nonetheless reliable, consider the hadith which says: “The greatest name of Allah has seventy three letters. There was only of these letters. He spoke that one letter and land between him and throne of the Queen of Sheba (Bilqis) sunk down as such that he could reach her throne with his hand and the land returned to the original state..." (Al Kafi). All of the hadith in this chapter are weak. However, if one looks to Basa'ir al Darajat, they will find almost a dozen different chains which demonstrate that this hadith was widespread; and in addition, they will find an authentic version of the hadith in Kamil al Ziarat.
If we were to list all of the examples of this, we would be writing for volumes. The point is just to illustrate that Al Kafi was not written haphazardly, but that it was written with an awareness of the principles of the sect.
Weak Hadith Are Still Useful
Weak hadith are still useful for a number of purposes. One purpose can be that they provide corroboration for a hadith. For example, the following hadith is authentic:
It is narrated from him from his father from Sulayman al-Ja’fary from abu al- Hassan al-Rida from his father (a.s.) who has said the following: “A people came to the Messenger of Allah during one of the armed expeditions and he asked who are these people? They said, ‘Believers, O Messenger of Allah.’ He then asked, ‘What is the degree of your belief?’ They replied, ‘It is patience in hardships, thankfulness in prosperous life and agreement with the decision of Allah.’ The Messenger of Allah then said, ‘These are forbearing and knowledgeable people. Their understanding is almost to the degree of the prophets. If you really are what you say you are, do not build what you will not use for dwelling, do not accumulate what you will not eat and be pious before Allah to whom you will return.’” (Al Kafi)
But it has another chain supporting it in another chapter. It is helpful to have this second chain even though the first chain is authentic and the second chain is weak just because it gives us more assurance that the Imam actually said it since we are getting independent attestations for the narration.
Another purpose for weak hadith is that not all of them are necessarily lies or wrong. There are many demonstrations of that fact above. However, there are even instances where narrations from weak narrators can be accepted. For example,
Abu Ali al-Ash’ari has narrated from Muhammad ibn Salim and Ahmad ibn abu ‘Abd Allah from his father all from Ahmad ibn al-Nadr from ‘Amr ibn Shamir from Jabir from abu Ja’far (a.s.) who has said the following: “O Jabir, do people think just claiming to be our Shi’a and saying they love us ( Ahl al-Bayt) is enough for their salvation? I swear by Allah, of our Shi’a are only those who are pious before Allah and obey Him. No one, O Jabir, would know them except by their humbleness, submissiveness, trustworthiness, their speaking frequently of Allah, fasting, prayer, virtuous deeds toward parents, good relations with neighbors, the needy, the destitute, the heavily indebted, the orphans, by their truthfulness, recitation of the Holy Quran, and restraining of their tongues except for the good of the people and their being trusted among the people of their tribe in all things... (Al Kafi)
This hadith is weak due to Amr b. Shimr, who was a Ghali. See Shiiticstudies, Worldview of the Ghulat (Part 1) (Part 2) However, the hadith is acceptable because the hadith actually goes against the beliefs of the Ghulat who believed that the Sharia is abrogated and that the Shia could go to heaven even if they are unremorseful sinners. Because of this, we can conclude that the hadith was probably faithfully recorded.
Yet another reason why weak hadith might be helpful is because they can help us understand nuance and illustrate the general principles found in authentic narrations. For example in "The Book of Intelligence and Ignorance" of Al Kafi, we find the following weak narration:
Ahmad ibn ’Idris has narrated from Muhammad ibn ‘Abd al-Jabbar, from certain persons of our people in a Marfu’ manner, who have ascribed it to abu ’Abdallah (a.s) who has said the following. The narrator has said, “I asked abu ‘Abdallah, “What is Intelligence?” He replied, ‘It is a fact with whose help one worships Allah, the Merciful, and attains paradise.’ (Al Kafi)
Although this narration is weak, it tells us the meaning of "aql" as found in the Shia hadith tradition—that the "aql" is the "organ" of a believer's obedience and disobedience to Allah and their understanding and capacity to fulfill Allah's commandments. Kulayni is more or less trying to tell the reader his understanding of the meaning of "aql" by quoting a narration like this. But one does not need that narration to actually get this understanding. If one simply ponders over the other authentic narrations in that chapter, they will come to the same conclusion:
Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad from ibn Mahbub from ‘Abdallah ibn Sinan who has said the following. “I mentioned a person, who would overdo the deeds of his doing Wudu and prayer to Imam abu ‘Abdallah (a.s) and added that he is a man of good intelligence.” The Imam (a.s) then said, ‘What kind of intelligence is it that allows him to obey Satan.’ I then asked the Imam, “How would he be considered as obeying Satan?” The Imam (a.s) said, “Ask him wherefrom this thing comes to him. He will certainly say, “It comes from Satan.” (Al Kafi)
What is obvious from this narration is that it is the aql which is what allows someone to obey Allah, and a deficiency in it is what makes them disobey Allah by obeying Shaytan's waswasa. So one could easily derive the meaning of the earlier narration about aql from this authentic one, but it is still helpful to have quoted the earlier one because it illustrates the principle more clearly and plainly without extra steps.
The Sources of Al Kafi
Something else that lends credibility to Al Kafi is that it is not a work compiled from oral sources. Rather, Al Kafi was written by collecting a number of written texts. For example, Kulayni often quotes from books written before him like Saffar's Basa'ir al Darajat and al Barqi's al Mahasin. But more than these major compilations, Kulayni was quoting from little notebooks written by the companions of the Imams themselves! Hossein Modarressi in his "Tradition and Survival" lists a number of the written works of these companions that Najashi and Tusi listed in their rijal books. Etan Kohlberg has also written a paper called "Al Usul al Arba'umia" where he outlines the history of Shia hadith compilation as well, noting that the bulk of the Shia hadith tradition rests on a network of written works that were passed down from one generation to the next.
The significance of the fact that Al Kafi and other Shia hadith are based on written works is that it means that there is more leeway with respect to the later narrators in the chains in Shia hadith. There is no doubt that being strict and careful, and concerning oneself with authenticity is important—however, for later narrators for which we do not have explicit authentications and only some praise for, it is conceivable that we can be rest-assured in relying on them. This is because by the time the narrations have reached these later narrators, the books they are narrating from have become famous and have been copied enough times for them to be textually stable and where fabrications could be caught more easily.
For example, although the narrator "al-Husayn bin al-Hasan bin Bundar al-Qummiy" does not have explicit authentication anywhere (and is therefore majhul), he is what is known as a "Shaykh Ijaaza," meaning that he is passively transmitting books which have become textually stable. In other words, we can rely on him because it is unlikely that someone can tamper with a well-known and widely-circulated book without being caught out. See ShiiticStudies, Bahth al Fihristi; Zanjani, The Authenticity of Ziyārat ʿĀshūrāʾ.
Yes, 60% of Al Kafi is weak. However, Al Kafi has more than 6000 authentic narrations. Even if one were to remove the weak narrations from the book, nothing of substance would be lost. One could still just as easily establish a full Shia aqida using only authentic hadith in Al Kafi. The reason for this is because almost all of the chapters in the book either have an authentic narration in them that anchors the meaning and general message of the chapter, or the narrations have support from authentic hadith in other books. So if we focus on the meanings and general concepts of the narrations, very little of the content and general meanings of Al Kafi is unsubstantiated. The reason why weak narrations are important to quote anyway is to show supporting evidence and extra historical details or nuance on the subject. Finally, because Al Kafi and other Shia hadith rely on earlier written sources, not all weaknesses in the chains of narrations are necessarily relevant. Regardless, while one should still be concerned with authenticity, a layman does not need a microscope over a rijal treatise to benefit from Al Kafi.
 I thank Sayf al Din al Hashemi from "The Believers" server for this beautiful phrase to describe Al Kafi