So I know that some schools have NBME questions on their exams and some don't. How helpful is it to have NBME questions on exams during M1 and M2 in preparation for the Step 1?
It's probably the single best thing a school can do for it's students. It basically allows students to prep solely for step 1 for 2 full years and removes the ability of professors to test you on minutia or clinically irrelevant material from their research/field of interest.