I recently started a job at a GWT shop, and I picked this book up on a friend's recommendation.
This book's strength is its comprehensive view into process of building a complex, interactive site using GWT as a tool. As a professional web developer, I'm less concerned with having someone teach me the syntax (because I can google for most of it) and more concerned with getting real-world advice about how to construct my application. Having an authoritative source to not only offer solutions for security (Acegi), builds (Maven) and persistence (Spring) but also show me how all of those things come together is an invaluable resource.
Anyways, if you don't have much web development experience and are interested in GWT, you will probably find books out there that more focus on a comprehensive technology-driven learning experience. On the other hand, if you are ready to learn about how a GWT application comes together from start to finish, if you want to know how to overcome specific challenges, I'd have no qualms about recommending this book to you.
Specifically, I found the sections on hibernate and google maps integration to be particularly valuable. I really only want to learn the aspect of those technologies that are relevant to GWT, and having the salient information handed to me on a platter saved me a lot of time. As a professional developer, the most valuable information I get is from asking my colleagues how they go about solving problems, and this reading this book was a lot like listening to some of the better answers I've gotten over the years.
Bottom line: If you'd like to sit down with an expert in GWT and get taught the nuances of the language from beginning to end, there are other books that specialize in that. If you wish you could sit down with an articulate, intelligent web developer and ask "How did you deal with X?", then this is the book for you.