Alright mayo, I am convinced. By now I have taken each GMATPrep CAT twice: 640, 610, 580 (after a week of not doing any problems, I was quite nervous), 630. My test is scheduled for July 18, and I think that I will gain a lot from doing MGMAT CATs from now on.
GMAC is also going to send me a CD with CATs on it; though I suspect that these CATs are the same as the ones I have already downloaded from the site.
Anyway, I'll update on what I think of the MGMAT. Thus far, I think that the questions are fairly representative with a few that are quite hard, which parallels what you said. However, these are questions that came from the question banks that come with each book (I have access to the four quant banks). This weekend, I'll take the first of six MGMAT CATs.