Each of the chapters of this book fall into one of three categories: an interview with a prominent game designer, an analysis of a successful game, or a discussion of game design principles. Each is valuable for different reasons, so I'd like to review each of them separately.
The interviews are interesting, and go into much greater depth than most interviews you see. As a result, you're able to get a good idea of how each designer approaches the game design process, which can be useful in analyzing your own methods.
The chapters analyzing games focused on a single game, but also looked at similar games in the genre. If you're an aspiring game designers, you'll benefit from these chapters. Partially because you will see what these games did well, but moreso because they will encourage you to analyze the design aspects of the games you're playing to see what they do right, what they do wrong, and how you can apply that to your own designs.
About half of the chapters of the book cover various aspects of game design, presenting the author's own theories about what's important, what isn't, and the things you should be thinking about. It's hard to review the value of this; some of it you'll agree with, and some of it you may not. Depending on your degree of experience, some of it may be obvious, some of it may be new, and some of it may help you focus on areas you've been neglecting.
Overall, I wouldn't consider this book a must-have, but if you're interested in becoming a well-rounded and successful game designer, there's a lot in here that will be of value to you.