This slim volume is the standard reference for research in automata theory, languages, and computation (especially regular and context-free languages). For that, it gets five stars. As a textbook for students, however, it is dense, uneven, and confusing throughout. Generations of novice computer scientists have been soured forever on theory by being forced to endure this book in their undergraduate- and graduate-level theory courses.
Conclusion: buy this book and keep it on your shelf, with the other essential references, but if you want to *learn* the material, look elsewhere -- for example, Michael Sipser's excellent new textbook, _Introduction to the Theory of Computation_.