Kaplan - tough and not reflective (according to the forum)
Princeton Review - Math super easy, Verbal is somewhat tricky and the answers aren't the best chosen (I disagreed with many)
MGMAT - math super hard, Verbal very similar, test score in the end very similar to final test score
GMATPrep - This is the REAL thing, so it is probably the most accurate. I heard the math isn't as hard as the real thing, but your score will most likely be.
PowerPrep - old GMATPrep by ETS. Math was a lot easier (I finished with 10 minutes to spare and got only 2 wrong - Q50). Verbal was about the same as GMATPrep/OG
Those are my contributions to this thread