This seems to be the
book on computation theory. As in, every time I looked up an example of a PSPACE-complete problem on the Internet, it was an example that Sipser gave in this book. It's good to know that I have the definitive book on the subject, but you've got to wonder about the domain's relevance if it can be totally summarized in one textbook. Maybe that's just the math snob in me talking though.
Overally, this is a good textbook with detailed proofs and fairly decent explanations. Sipser was a better computable functions teacher than my prof was, which is saying something.