I loved taking practice exams because they would force me to sit down and actually work on my GMAT for a period of time without being distracted. When I just did books, or other questions, I always got distracted, but taking the practice exams, I knew I had to finish it before I could know the score.

Having said that, I would probably still recommend not doing them in great excess too soon, because there's only so many of them. If you haven't yet, you might want to take a GMATPrep test now, to see your starting point, just as a gauge. But in terms of starting a regular routine of prep tests, I would probably suggest starting after you've finished all your MGMAT books.

My take on prep tests is to use the GMATPrep for gauging progress, and all the other companies' tests for content. Meaning, like I first said, to take them to practice question types and identify weak areas. So even after you've started the regular routine of prep tests, take the GMATPrep sparingly to measure your progress, but keep in mind you can always retake them. There will be some repeats, but a good part of the questions will be new. Whenever you are studying, make sure to note down the principles and areas that you are weak in so that you can review them later.