Great book. Not just for those interested in DSLs, but also just to become an advanced Groovy programmer and see how some libraries you use every day (like grails and markupbuilder) actually are implemented.
I didn't finish this book. But that had more to do with time constraints than with the quality of the book. This book outlined very nicely how to build a DSL in Groovy. It was a great help on the project I was working on.