I teach the MCAT for kaplan and I can assure you the diag is pretty darn valid. Usually people dont study for it at all, which is the point. Its not that the test is biased towards you getting a poor score, its just that you cant recall everything from 3 years of undergrad without practice - and thus you pay for the course. I have to admit, I shelled out 1000 for course back in my day as a premed, and I'm such a slacker that if I hadnt, I wouldnt have put in the effort to studying for the MCAT ( and subsequently blasting it out of the park). I had a 24 to start with and studied my balls off for 3 months to push that up to a 36. All the STP crap that I teach in the MCAT classes...It really does work....most of the testing strategies I learned for the MCAT are the same things I'm learning to get ready for the USMLE.
FYI - if you guys do well on the MCAT do teach for kaplan in med school cause they pay for a lot of your USMLE prep when you work for them.