Took the GMAT yesterday and got a 780 (M50 V48). A few impressions:
1. The GMATPrep software is pretty accurate - got a 770 and 780 on those practice tests. I would also say that they fairly accurately mimic the "feel" of the test in terms of timing and types of problems.
2. MGMAT's practice tests are pretty accurate too. (I took all six, then reset it and took all six again). I started out with a couple of 730's but I did not finish those in time. As I got more serious about timing I scored in the 770-800 range on MGMAT tests.
3. But MGMAT's math section is harder to finish in time than the real thing. I had a lot more time on the real test yesterday, and looking back, I wish I had spent some more time on the earlier problems. I knew how to do every problem I saw, so in order to get a 50 I had to get a few wrong. And since I knew how to do them, I can only conclude that I made careless mistakes early in the test because I was rushing. Later in the test, when I realized I was ahead of schedule, I took more time and probaby got all those questions right.
Obviously I'm happy with my score and I think MGMAT's tests are great overall (thanks!), but one tweak I would highly recommend is that MGMAT make the timing more realistic so that people aren't in the habit of rushing on the real thing.