William C Wake
That said, it's the differences which matter. .
GoF chose mainly UML and C++ to describe the patterns in detail. This book considers how these patterns can be used in Java. That in itself is significant (at least if you use Java), but there is also a strong difference in style between the two books..
GoF is dry and academic, sometimes requiring a lot of study to understand and apply abstract pattern descriptions. This book, on the other hand, is much more lively - full of worked examples, self-test problems and puzzles, all set in the context of a fictional firework manufacturer. This makes it a much more enjoyable read. And if you get stuck, the solutions are all in the back..
However, this easy-reading, approach has a negative side. The patterns are so comfortable and copyable that subtlety can be missed. Take Flyweight, for example. Admittedly this is one of the rarer patterns in the wild, but in this book it seemed more like a recipe for one particular usage than an exploration of possibilities..
In summary, GoF is a crusty classic worthy of anyone's bookshelf, but this book provides a useful and practical introductory step for anyone working with Java.
(Frank Carver - Sheriff, May 2006)