I just passed the 70-646 exam with an 875, so that should tell you something about this kit. First of all, you have to not only study the book, but use all the included resources - questions at the end of the chapter, the practice labs (I used a virtual server), and of course the practice tests on the CD. There's no substitute for hands-on training. Reading this book alone doesn't guarantee a pass, nor should it. You have to use the product, understand the concepts, and shore up the weak areas you'll discover when you do the practice tests.
There is some material in the book that's a little overkill. For example, there wasn't one question on the exam about IPv6, even though the chapter on this goes into a lot of detail. There also wasn't much on the exam about the appplications within IIS. I didn't mind though. since my next exam WILL cover these. Learning more is always a good thing.
All in all, this is a pretty thoroughly written book for the exam objectives. I've gone though a couple dozen certification books over the years (including others by Ian and Orin), and this one rises to the top as one of the better ones.
Thanks for the pass guys!