Take a look at my post a few threads down ( "770 my first try" - the forum won't let me post links yet). The
MGMAT was very accurate for me. I was really doubting it as I didin't think I was capable of the scores it was telling me, but it lined up to my GMATPrep & GMAT results. IMO they are a great set of CATs and I recommend them to everyone.
The math is a little harder than the real test, but the score it spits out correlates to the GMAT well. The verbal is a little easier, but again it correlates to your GMAT score.