I used the Kaplan one (this was more than 6 years back so times may have changed).
I found that it did ok on the mathematics portion, you either know definitions and how to apply them or you don't. I felt it did a good job of covering the mathematics topics that were asked on the exam.
Writing was rather basic, I got something like 5.5/6.0 and I wrote an essay in the same format that I did for my public high school English class. 5 paragraphs, 5 sentences each paragraph, with no fancy words. Books usually don't have much to say about these.
I felt that it lacked on preparing me for the verbal questions. Maybe it was just the way I studied, but I recall the book focusing on the "most frequently missed words" and it wasn't really of much use. I do recall being surprised by the questions in the verbal -- i.e., it had not prepared me for the types of question I would receive (or at least, not the frequency or severity).
I think that it concentrated some time on test taking strategies. Back then it was based on a tier system. If you miss the first question, you were more or less put into the 400 or below bracket (with some chance to creep back up if you answer many more correctly, as it bases the difficulty of the next question on the correctness of the previous answer). I recall that was helpful to at least know.
Will you be taking a subject exam also?
