Instruct: Classify Amazon reviews into positive or negative sentiment
Query: Some problems, but otherwise excellent practice

I bought both Kaplan's and the Official GMAT prep books, and they complemented each other well. The Kaplan CD gives you a sense of what the mechanics of the CAT are like and the ETS Official Guide shows you what to the questions are really like. That said, I owe my 750 to the Kaplan CD (although the practice tests are significantly more difficult; I scored 600, 650, and 640 on the practice tests).