Due to my experience I can say that most of the practice tests from the usual suspects (Kaplan, Veritas,
Manhattan Prep, etc) are fairly good predictors of GMAT scores. Gmat Prep has to be at least as good as them since it features "real questions". However, I did find that the GMAT Prep Practice tests were considerably easier than all the other practice tests I took. Gmat Prep Practice Tests were also better since in my experience they were the most similar to the real GMAT.
If you're taking the practice tests to gauge your potential score keep in mind that a practice test can only give you a good score prediction if you take as many tests as possible to rule out any false results and also if they are taken in real-test conditions. Try to mimic real test conditions by always taking and completing all sections of the exam to the best of your abilities and by timing yourself as if you were taking the real GMAT. Keep in mind that scores of practice tests that you have already taken may not be good predictors since you've already answered those questions before.
Practice as many problems as you can and review and redo all problems that you get wrong and you'll keep improving!