Ive spent a lot of time studying both the GMAX and the material from the American Education Aids, Chennai.
I believe the latter to be far more comprehensive and close to the real thing. The amount of practice questions that you get is just awesome. Besides, it also offers you the Powerprep software and 2 PBTs for free.
Another distinct advantage of this material is that each section on the GMAT is split into two categories, one where you build foundation and two, where you hone your skills.
Not to mention, the 90 issue and argument sample essays to crack the AWA section.
I owe a lot of the credit for my 720 to the Chennai material. Besides, its a lot
cheaper!