My biggest complaint with this book is that the authors basically just took the design patterns found in Java and C++ and re-implemented them to run under ActionScript 3 (AS3). The list is comprehensive, but it's clear that the authors don't "think in AS3".
In several core ways, AS3 is very different than Java and even more so with respect to C++. For instance, the event model is baked into the language and asynchronous programming is a different style. Also, XML and XPath are native constructs in ActionScript 3, not libraries like they are in other languages. These differences (among others) imply that some of the original Gang of Four (GoF) and Java patterns manifest themselves differently and some patterns don't apply at all. There are a few places in the book where the authors use the built-in events infrastructure and few other native features, but it's clear that they don't think in AS3. It seems like they think in Java.
For instance, the observer pattern is one of the core GoF and HeadFirst patterns. However, the native event capability in AS3 serves the same purpose. Rather than show you how/why to use the native event capability, this book happily shows you an AS3 translation of the GoF/HeadFirst observer pattern and never tells you to use the built-in event capability instead. In contrast, the Joey Lott and Danny Patterson book from Adobe Press, does not have a section on the observer pattern, but there is a chapter on "WORKING WITH EVENTS".
The above problem would be enough for me to recommend that you not buy this book but it gets worse. This book is not even great at teaching you how to think in design patterns. To be fair, neither is the original GoF design patterns book nor is the Lott/Patterson book. The best book for this purpose is the HeadFirst book. Its examples are Java but, the HeadFirst book walks you through application evolution which really makes the case for why the patterns are useful. The HeadFirst book also includes exercises and discussion as well as a quirky style that really make the concepts sink in so you learn to recognize when to use each pattern.
If you are an AS3 programmer who is already familiar with design patterns, just get the Lott/Patterson book. If you are new to design patterns, get the HeadFirst book AND the Lott/Patterson book.