These are the best practice tests you can take - which should be obvious since they're made by GMAC. They are the best representation of the pacing and progression of the real test and my scores on these tests most consistently matched my actual test performance.
If you do this, I also suggest you purchase the other exam pack. It's equally as good and just helps provide more practice. I always think for these tests the more questions you see, the better. I don't remember how much these cost, but they are definitely the most valuable practice tests and should be the first materials purchased.
I took a couple tests early in my studies as a benchmark, then moved to other sources (Manhattan, Magoosh), then finished up with the two or three I had left the week before my test to solidify timing.