It seems to depend on the test taker, although most people seem to improve a lot on the real thing versus the MGMAT cat, especially in the quant area.
Personally, I improved in both areas. These are my results so far:
640 (Q40, V41)
3/22/2008 MGMAT 690 (Q38, V45)
3/29/2008 PR 660 (Q43, V39)
4/13/2008 MGMAT 640 (Q40, V37)
4/26/2008 MGMAT 650 (Q44, V35)
5/11/2008 GMATPrep 740 (Q46, V46)
5/29/2008 MGMAT 430 (Q44, V4) (skipped verbal)
5/31/2008 GMATPrep 760 (Q49, V45)
Any idea as where to download the PR(Preview Review) ? Please guide.