I use it, and also have IdentityGuard, which is at a discount from Costco. CK isn't quite as good as the paid service, but seems to estimate some of the other related credit scores pretty well, and is close enough on FICO that it's a good ballpark.
I don't disagree with this...
But, people need to realize that their FICO or any other score will change daily, sometimes by 10-30 points or more depending on various factors that might change. Each creditor also creates their own score based on the underlying data.
A few years ago we had an "incident" that we dealt with. Long story short - a very large national collection firm placed a fraudulent mark on our credit - not a mistake or anything that had anything to do with us. This was an outright criminal act by an executive officer of the firm in retaliation for a complaint we filed that cost his firm big bucks and him his job.
In the process of getting it cleared up...which ultimately took about 3 days because my wife was tenacious about it and we were in the right AND had evidence of bot a crime and a civil infraction that went way beyond just a violation of the Fair Credit Reporting Act...we were having credit pulled every 2-3 hours with the assistance of one of the bureaus. I think it was Experian but basically they could do an administrative pull which is just like a hard pull but does not create any record that is visible to anyone except the firm itself. It is the kind of thing used for investigations and for quality checks. I will say that the bureau was extraordinarily helpful with this. At first they sort of gave us the brush off but when my wife got a letter from a judge asking for their help they because very cooperative.
Our credit scores changed by almost 100 points both up and down over the 2 days while the bureau made the corrections needed to purge the illicit information. Most of the variations were minor as they adjusted outstanding balances, credit access, and that sort of thing. But, still, these are things that normally happen every day to everyone depending on when changes to your accounts post. So basically, 20-30 points on your credit report is noise that is not worth worrying about. And CK has, at least for us, always been more accurate that 20-30 points. I'm sure it could be different for others but if you are just looking to get an estimate of whether your credit is good or needs improvement, I think CK is an excellent resource.
(I have no affiliation with CK and am only expressing my person opinion.)