I am doing it. I have banked everything we've covered so far as we've gone over it in class, which has me at a little over 10% right now. I've found it to be super helpful for me so far, but BABSstudent is right in that sometimes what's on the cards will be more (i.e. clinical correlations of the physiology that we cover this year) than what you were taught in class so you'll end up either learning extra stuff or not banking that card yet. For the most part thought that aspect of it hasn't become too bad; I don't know if I'm lucky or what but most of the stuff we've been taught so far can be found straight in the FC cards, so it's both Step 1 (I guess) and class review for me. It is amazing how much more I remember than my classmates from our first few months of class, and it's actually been a help on exams because I remember most of the stuff we learned four weeks ago rather than having totally forgotten it in the intervening time.
It can be a burden, though. The time doesn't bother me because I view it as studying for class rather than something extra, but if your school stuff isn't covered that well in FC it would be annoying. So in my case we've been plowing through upper and lower limb anatomy with brachial plexus and sacral plexus. My study quiz today is 181 questions, which will probably take a little over an hour. Which reminds me, FC only works if you're the type of person who can look at a card with info you didn't remember, try to understand what's going on, and then move on quickly. If you're going to thoroughly review everything you don't remember there's no way it's a good investment of time; you just have to trust the spaced repetition to work, which I have found largely successful so far.