I should have added a disclaimer: "most people hate the Purple Book, I'm one of the 2% who thinks it has a place".
Helio, nathan12343, and dlenmn are quite right. The book sucks and is hated by almost everybody. If you use the book expecting to have a "good" or "comprehensive" study guide, you'll be gravely disappointed! In fact, this book made me give up all hope for the PGRE. When I first started studying, I had the ETS book and the Purple Book. I tried to start with the practice exams from the Purple Book, but I quickly despaired and gave up studying altogether. When I finally started looking at the actual practice exams a few weeks before the test, I realize that it was doable, but by this point, it was too late to do any serious studying (thanks to a tough fall semester). I ended up getting a 780.
When I started studying again for the second time, I grudgingly decided to give the Purple Book (and the easier "Physics Builder" also published by REA) a second chance. Working through the selected problems
, helped me get a lot faster at working multiple choice physics questions, and using the ETS practice exams, I was able to discover and patch the holes in my knowledge. This study method resulted in a 200 pt increase up to a 980.
If you do end up using the Purple Book, never get hung up on a question. If your answer disagrees with the solution, there's about a 50% chance that the solution is wrong! At first I found this extremely frustrating!
In summary: don't put too much reliance on the Purple Book, but still recognize that for all it's faults, some people (myself included) have found it to be useful on occasion.