My first book was Kaplan. This book contains 2 full practice tests with an additional test online. I found the math sections to lack much actual review of math concepts. I hadn't studied math since high school, and using this book made me feel like I should know much more than I did which made me feel very nervous.
The verbal practice is very challenging in this book. At first, the difficulty really harmed my morale. But all the other verbal practice I did seemed easier in comparison.
The essay strategies in this book are much better than the Baron's book.
The essay review is kind of lacking. The verbal was easy throughout the book, and then the practice tests were pretty hard. Ultimately, I thought the practice tests were super hard, and lowered my morale.
Interestingly, both Baron's and Kaplan had more than one mistake in either problems or answers that made me generally suspicious as I was checking answers. I think these books will improve as people get used to the Revised test.
Other free websites I found helpful: Endless Reading Comprehension Practice, Free Practice sets, Magoosh GRE Prep (I found some of the free lessons helpful like for example, the one on exponent rules.)