Hi angarg,
The scoring algorithm on the Official GMAT is far more complicated than most people realize. Since that algorithm is proprietary, no GMAT company has an exact match for it, thus CAT scores can vary a bit based on the 'biases' involved in their respective designs. The most realistic CATs available are the 6 from GMAC (the 2 free CATs, and each of the 2 that come with Exam Pack 1 and Exam Pack 2), but the CATs from Kaplan,
MGMAT and Veritas are all 'close enough' to the real thing that they will provide you with a relatively realistic score assessment (assuming that you use the CAT correctly).
While there are certain aspects to Test Day that you can really only learn by taking realistic, FULL-LENGTH CATs, once you've learned those things, it's important to remember that taking lots of CATs will NOT make you a better Test Taker. A CAT is really a 'measuring device' - when used correctly, it will give you a realistic score and help define your strengths and weaknesses, but it will NOT help you to fix any of those weaknesses. To raise your scores, you have to learn the necessary Tactics and put in the proper practice and repetitions. The CAT will show you whether your studies are helping you to improve or not.
1) How long have you studied?
2) What study materials have you used so far?
3) How have you scored on EACH of your CATs (including the Quant and Verbal Scaled Scores for EACH)?
4) What is your goal score?
5) When are you planning to take the GMAT?
GMAT assassins aren't born, they're made,
Rich