a GMAC product; it's the older version of GMATPrep. There's a difference between the two, however- GMATPrep clearly has a much larger pool of very difficult questions. This means that on GMATPrep, you can get a lot of questions wrong and still get a very good score, just as long as you're only answering the supremely difficult questions incorrectly. On PowerPrep, the hardest questions seem to be only medium-difficult level, so if you get a couple of these wrong, the test becomes convinced that you're not a top (50/51 level) scorer. The underlying scoring algorithm should be more or less the same, but the question pool is quite different.
Nov 2011: After years of development, I am now making my advanced Quant books and high-level problem sets available for sale. Contact me at ianstewartgmat at gmail.com for details.
Private GMAT Tutor based in Toronto