G912,
I just took the April MCAT and I would say the AAMC III-VI test are a good investment. They are closer to the real thing than any of the commercial tests available out there. Sure, the format is a little different, but no test is exactly the same as the real thing. Also, from what I can discern from different posts on SDN, AAMC IV-VI give you the closest approximation of how well you'll do on the real exam date. Plus, the computer will break down your strength and weaknesses so you may concentrate your studies.
Good Luck and make sure your do tons of practice problems (almost more important than reviewing the science itself).