I just got done taking the GMAT this morning. I started preparing about 2 1/2 weeks ago. I took Kaplan,
MGMAT, and the GMAT PREP (in addition to a lot of practice problems on here). By far the most accurate to the real deal is the GMAT prep. I was scoring 650ish on the first two, but between 680-720 on the gmat prep and received a 710 (49 quant/38 verbal) on the real deal. The questions and the length of time per quest required to complete (especially math) was much closer to the GMAT Prep software. The math questions on this board, for example, are way too time consuming with quant compared to the real thing. I didn't have any probability or combination problems at all, which was surprising. I had far more geometry based questions than I would have liked.
My advice for anyone trying to prepare for this in a short time frame like I did:
1) touch up on your basic algebra and geometry skills
2) take a practice test to see where you are with your score
3) read one or two gmat books (I read Kaplans--it was decent on verbal but just ok for quant)
4) take as many practice exams as you can and review those answers that you got wrong.
My first gmat prep I got a 620 so I got it bumped up 90 points basically in less than 3 weeks (scored 45/28 on first practice exam).
Hope that helps!