Hey guys just wanted to share my FC experience, since I just got my Step I results back, and it went well.
I used GT/FC from the summer between MS1 and MS2 and onward up until test day. My first priority was to flag physiology (is/will be relevant through all other subjects, so IMO probably the most important to know), biochemistry (heavy memorization will gain the most from long term, repeated review), and microbiology/immunology (same reason).
I would have aimed for Anatomy too but there were a looooooooooooot of those and MS2 already started by this time. Looking back though if you are fresh with Anatomy then do those. They should be easy because a lot of them are really redundant (a million cards asking for actions and innervations of muscles) and you can get rid of them via perfect recall.
Anyways, by the time I was done with MS2 and in dedicated prep time, I flagged about 67% of the total load. Most people will tell you to drop FC by this time, but I'm a bit OCD about these things so I just kept using it. This is very risky, because I spent hours just going through my daily review. I flagged massive amounts of topics as I went through reviews of subjects at this time, with the peak of one day having a question load of about 500-600 questions. The majority of which I could fly through because it was the hundredth time I've seen them, but still...that's a huge time sink. And many people will tell you to just stick to FA and UW. For me it was more like FC and UW (see below).
The reason I took this risk is because I felt like I was retaining information throughout the year, it showed well in my grades, I did well on a school-administered NBME when the majority of the class did not (since it was pre-dedicated study time for most people), and I was already averaging well on UW from the beginning (78-80%). Basically, it seemed to be working. So I put my faith in it and it paid off. If you count time spent on it, FC was actually my primary source, above FA and UW, even though I was cross-referencing across all three, and placing emphasis and new findings on UW. Technically, when I flag cards I annotate them into FA too so I was kind of looking FA over at the same time, but I never read it cover to cover. FC has the majority of good points from a mix of sources, including lots of stuff straight out of FA, often with better mnemonics (seriously, "Copper is Hella Bad" in FA for Wilson's disease is probably the worst mnemonic ever). The only things missing that I found to come up on UW were tiny details in FA (e.g. pertechnetate used to detect ectopic gastric mucosa, for Meckel's diverticulum). But FC has come a long way since I started it, and you can make your own cards and topics now (I never used this feature, even when it was implemented).
I guess the take-home point and advice for using FC is firstly to try with all your might to finish flagging before study time, that way you don't have to sacrifice other things like I did and still have a small daily review each day. This was actually my goal from the beginning, but FC was just too time consuming and I am too obsessive haha. e.g. I planned during dedicated study time to go over each system in FA again but I actually did not (besides Cardiology, GIT, Repro, or other HY/weak areas). This is obviously a bad idea, but it turns out 1.) I was already strong in that area, as evidenced by class performance and UW stats 2.) I still remembered those systems because I've been through FC so diligently.
good luck guys