I used it for both the shelf exams and then for my specialty exams (much, much harder).
Pros: LOTS of questions with a lot of detailed explanations.
Cons: I'm guessing 20% of the content in the qbank was not on the actual exam or more fringe topics. The questions can also be a little too difficult for the NBME level.
I would suggest it, but use it in addition to the self assessment resources. In general, FM Shelf isn't too hard, just go through practice questions and you'll be fine.
