I'm not a native speaker of English and have never taken a standardized test before in my life; yet I scored a 770 (99 percentile) on the GMAT in my first try 3 days ago.
I started out with Arco's 24-hour course. Arco's book is great, and it has quite a bit more stuff on the math and verbal part than Kaplan has. I didn't get the CD rom though since I got an older edition, so I can't rate the software. But the book itself is excellent.
Arco has a lot of useful info on both the math and verbal sections. There are a lot of relevant problems in the book, and their style comes close to the real thing. The book is well written with lots of useful advice. There a very few mistakes and printing errors in the book.
After Arco I went on to tackle the official guide from ETS. Anyone who wants to prepare for the GMAT should work some of the problems in this book. I did them all, but I don't really think that's neccesary -- just do the final third in each section and you'll be fine.
Finally I read the Kaplan book. There was not much in the book that I didn't already know. In fact, Kaplan has much less material than Arco. However, when you start working with Kaplan's CD rom you realize how much info is packed on that disk. There's 18 quizzes with 9 math and 9 verbal and they're all great practice. They actually force you to work *faster* than you'll need for the real test. Also, there are very few errors in the Kaplan material, and their problems come very close to the real ones.
I took the four GMAT CATs on the CD rom and got 700, 780, 660 and 700. There's no question that Kaplan artificially lower your score so you're guaranteed to score higher on the real test, but that's cool as long as it doesn't discourages you. Just know that's the way it is.
I also took the PowerPrep tests, just for practice... but I had already done the problems in the ETS book ! However you definitely should work with PowerPrep to get a sense of the problems and the way they're presented.
Hope this helps. Good luck !!
In addition, I used Kaplan's CD and the GMAT PowerPrep software, both were also excellent.