The recently published 14th edition of Barron's Graduate Record Exam test prep manual is now available with a brand new CD-ROM that features an entirely new and up-to-date model computer adaptive exam. That is the way in which the actual GRE test is now given, which makes Barron's CD-ROM the closest possible simulation of test-taking conditions available anywhere. The CD-ROM presents an adaptive test that is unique to each test-taker, based on the way the first question is answered. All model test questions are answered and explained, and exams are scored automatically.
I picked this book up on the recommendation of a friend. I also tried a couple of books from other test prep companies (primarily when I had learned most of the questions on Barron's CD-ROM). Based on what I've seen, Barron's is far and away the best test preparation if you are willing to sit down and actually learn the material. I bought it primarily for math prep, and it focused heavily on underlying understanding of algebra and geometry. This book devotes comparatively little space to tricks and tips on how to answer questions you don't know, and seems instead to focus on making sure you know the answer. This can be more work, but in the end I think it's worth it.
Good for vocab (though there are FAR more words stuffed into this book than you'll ever see on the actual test). Be prepared- in all regards- for over-preparation with this book.