I read this book from cover to cover, not because it was required by the Formal Languages course that I took, but because it is a very good read. This book gives all the necessary details in every theorem that it proves, which can be considered both a good and a bad thing, depending on your level of knowledge of the subject. I personally believe it is a good thing, because after reading any proof in this book you do not feel skeptical if it works or not, like it can be after reading a proof that skips lots of steps.
I haven't used their online resources, and I didn't do many problems from this book, because the professor teaching the course came up with problems of his own. However, from what I've seen, they have a very reasonable collection of problems suited for self-study. Every well established field has a list of standard problem, and Language Theory is no exception. The problems in this book certainly cover most of the standard ones. Please, also be aware that although this book is a good read, it is not necessary an easy read - be prepared to invest considerable amount of time into this book.
I cannot give this book 5 stars simply because I do not think it is much better than previous editions. As a matter of fact, I think it is worse. I did not have a very close look at previous editions, but I know for a fact that they were more rigorous and formal and covered more topics. As a result of that, they were less suited for teaching an introductory course, but some of the topics they studied there are really nice. From what I've heard about previous editions though, it seems that they described several open problems, that are no longer open. So I'd suggest getting this new edition, simply because it has more contemporary information.
Some people write in their review that this book requires solid background in the area it covers. I respectfully disagree - I had little to none background in Language Theory and Complexity Theory prior to taking this course and (consequently) starting to read this book; however, I did very well in the course and enjoyed it very much. Of course, I was lucky to have an excellent professor teaching that course. If good books came with good professors that would be a killer package, but unfortunately they don't.
I have not read any other books in this field, so I have nothing to compare with, but all in all, for me it worked great and if you have a good professor and are passionate about the subject, I'm sure this book won't be a miss.