I took the Veritas full length (42 hrs.) classroom course for my preparation and was given access to their 15 CATs, five of which were Veritas Prep tests. The rest were from Arco, 800score etc. However, all these tests have their share of problems regarding markedly different level of questions and significantly different scoring algorithms. Based on my experience, I will suggest that you take as many practice CATs from as many companies as you like, but only for practice sake. Rely only on the GMATPrep tests for scoring. The GMATPrep tests will be the closest you can get to the questions on the real test and the scores will closely reflect your readiness.

I wholeheartedly agree with becnil, who always seems to know what he's talking about.

After the GMATPrep, my second-favorite tests are the MGMAT, but I think that the math is much more useful than the verbal. It's extremely difficult for test-prep companies to accurately copy the exact style of GMAT verbal questions, and I think it's best to use "non-official" verbal practice tests very sparingly.