I've never taken the GMAT, but several of my friends have, and it seems to be a toss up between Kaplan and The Princeton Review. Apparently Kaplan offers more difficult exams, which may better prepare students for the exam, but the Princeton Review book often explains certain concepts better. They both offer online and in-person tutoring.
I preferred Princeton over Kaplan because I felt they took more time to explain the concepts/understanding better. Really, you are better off getting the official GMAT practice book from the testmakers and slamming through them.
I keep hearing great things about Kaplan and spoke with a local office about a 6 Sigma course they are teaching. If you are not going to do any tutoring, I suggest the Kaplan book. However, if you are planning to take their class why not pick up another book and then discover the Kaplan book with the rest of the class (my plan).
After sitting down with a number of the books and flipping through them, I decided to start with "GMAT for Dummies"... also I find it a bit funny that a dummy would be learning about the GMAT.
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