I can only speak from my experience, but I just finished the GMAT about one hour ago and I feel like the only tests that are really close to accurate are the GMAC free practice tests.
I feel like the Veritas/Thompson practice tests are far too easy in the quant sections. Also, I scored an average of eight points lower on Veritas' verbal than I scored on both the actual GMAT and the other practice tests. There was something about the Veritas verbal sections that didn't agree with me.
The GMAC practice tests--the free ones you can download from http://www.mba.com
were the most similar to my GMAT experience. The questions were of similar difficulty and the scores were pretty close.
I hope this helps.