I had some mixed feelings about this one after going through it cover to cover, but when I reflected on it, I appreciated the author's hard work putting it all together.
As a book, it covers (almost to the point of beating to death) why and how Flight Simulator (2004 and FSX) can be used as a training aid for flying... hence the title. Once you've read it, you've either been proselytized into the fold, or you've gotten annoyed and thrown it into the trash- in either case, I can't see reading it again unless I wanted to dig up the arguments to have with someone else. The text is also a little more oriented towards the instructor, more than the virtual pilot.
But where the real value lies is in the CD. Here, Williams has provided dozens and dozens of open ended practice flights for a student or instructor to practice various techniques (in both FS2004 and FSX formats). Yes, you could make your own, or maybe find some on the Net, but he's saved you the time - just copy over, load up, review the charts and briefings, and start flying. Each flight also comes with flight descriptions/briefings on the CD- though not in printed form (save for a few samples that in the book). There are also a lot of online references (e.g. pointers to the FAR/AIM online, and some relevant chapters on the CD in PDF form, etc)- nothing you couldn't get online, but at least here the pointers are all collected together. Still, to me all that was worth far more than the price of the book.
If you're a raw beginner and/or you just bought FSX the other day and are looking to learn how to fly from scratch, you'll probably need to look elsewhere first before coming here. This book won't tell you on the fly what you did wrong while practicing, and you'll probably need to be at least a little comfortable with configuring FS yourself (in particular, messing around with the subdirectories in your FS program installation directory). But if you're serious about practicing technique beyond mere sightseeing and you know enough about technique and you're enthusiastic enough to self-practice, this is a good book for you.