Very good and understandable book if you had a first course in theoretical computer science. Many people say that the book is too hard, but if you are familiar with finite state machines (and especially some theoretical background) the parts they consider as hard are pretty easy though.
I would recommend this book eyery person, who takes a undergraduate course in compiler design. For example at ETH in zurich the course teaches big parts of the book.
The only drawback of the book is that there is no dragon on the cover. :)