I have done AAMC 3 and 4. I found on 4 that the score charting is very helpful. You are able to see what a 'difficult' question is and what an 'easy' question is. Then you can see how you faired against other students by section and by passage. Very helpful stuff.
gotta get all the easy ones and most moderate that gets you a 8-9 get some difficult ones and your score shoots up.
I've heard from some of my friends who took the test last spring, that the aamc items provide the best prep. I'm going through all the practice items right now b/c I think this is the best way to learn the language of the mcat... the practice passages have the same type of questions that the tests do, the hidden, go beyond the passage type... I figure if I master these (finish and understand my mistakes) I should be well prepared-