Don't be mislead by the name of this book, it is not "Design Patterns in Games" it is "Patterns in Game Design." Many of the people below have misread the title and have bought it before doing their research.
This is a book on game design, on a method of formal analysis that needs more attention in the game design world. Some people are afraid that if we list all game elements to pick and choose from, we'd end up with stale mass produced games. This is NOT true, please don't be mislead by this thought. Not only does that thought not prevent stale badly made mass produced games to begin with, but like many other structured artistic works (TV, Literature, and the like) games can benefit from a defined lexicon of elements (Patterns in this book). Recognizing the things that games share in common with each other can help you to find what it is that makes a game unique.
Like literature, everything under the sun has pretty much been done already in games. While there are some elements that have yet to be discovered in games, no matter what communication between Designer and Player still requires familiar elements to encourage play and game mastery. Because of this, there will likely be no completely contrary game. A game that is completely different from all other games (I mean shares NO elements in common with any other game), is likely to be a bad one. Just like a movie that shared no elements in common with all other movies would likely be unsuccessful.
Simply because these elements exist and are strictly defined does not require you to follow them strictly, the point is to use the pattern as a starting point to adapt, combine and create using the components given to you, much like programming you can use these abstract tools to create things of great complexity. The elements in the book are described thoroughly and give links to other elements that are related as well as problems and rewards that may arise with using the element. This is essential to understanding that design choices in gaming usually have inherent positives and drawbacks.
In conclusion, this book provides an excellent resource to the abstract art that is Game Design [...]. As someone who has used this method to analyze a particularly difficult to categorize game, I found the experience very rewarding, because I better understood the effect that each pattern gave to the game as a whole. There is one negative however, this book is a little old and new elements need to be added, but it can't go anywhere if the only reviews on it represent a mistake in purchase.
PS. My Game Design teacher at Digipen recommended that a good exercise for Game Designers would be to pick element patterns at random and try to make a good game out of that (remember you can adapt the patterns, they are only a start).