Let me start by saying that of all the books you should buy in order to best prepare yourself for the GMAT, this book by Kaplan should *not* be one of them.
Instead, buy "The Insider's Guide to the GMAT CAT" to learn all the skills and tricks. Then buy the "Official Guide to the GMAT" for more practice problems. Combine these two books, possibly also with the "Kaplan GMAT Exam Verbal Workbook" by Ingrid Multhopp, and the free PowerPrep software you can download from the actual testing folks and you'll be good to go.
Now let me explain why I think you *shouldn't* by this book: simply put, the questions in Kaplan's quantitative tests are not representative of what you will face on the actual GMAT, nor are the scores Kaplan gives you representative of what you can/will achieve on the real deal. Consequently, the Kaplan CD tests act as confidence deflators, and that's the last thing you need coming into such an important test as the GMAT.
Kaplan clearly stacks the decks with relatively harder questions so as to scare you into believing that you'll be in a world of hurt come GMAT test day.... unless, of course, you seek out "real" test prep services. And wouldn't you know it, Kaplan is in that business and they'd be more than happy to help you.... for a price. A price of some $.
I read the reviews for this book before I bought it and thought to myself, "how hard can the questions be?... it's just arithmetic, algebra, and geometry... perhaps having harder questions will better prepare me." Not so. They actually hurt you. Now by "hard" I don't mean the math itself is difficult; instead, it's the amount of math you have to do to get a solution combined with the effort it takes to get the root information out of the actual question which makes it hard. If you weren't timed for the questions they'd be easy, but with a limit of some 90s per question you can dig yourself a time deficit really fast on Kaplan's tests.
If the real GMAT isn't like that, why practice like that? Remember, realistically you only have one shot at the GMAT... do what is logical to best prepare for it.
I ended up getting a 710 on the actual test. This jived perfectly with the PowerPrep software provided by the GMAT folks and was just 20 points off the scores I was getting from the Official Guide and the Insider's Guide. It was 80 points away from the Kaplan tests. You do the math.