If you are looking for a book that showcases a bunch of existing Flex components (that aren't a part of the standard Flex framework) then get this book and enjoy it. The book is well written and organized, and Herrington does an excellent job covering a lot of really cool components that will have people who visit your site asking, 'Wow, how'd he do that?'
If you'd like to give a more in depth answer to that question than, "I just tied this component into my app" OR if you're looking for a book that deals more in how to create cool, advanced Flex components (like I was); then you might feel a tinge of disappointment. Chapter 15 is dedicated to this topic, but it will feel like child's play to anyone who has extended UIComponent.
To anyone who has played around with the "Tour de Flex" application (http://flex.org/tour), many of the examples will seem familiar. In fact, if you enjoyed this book you'll probably enjoy exploring Tour de Flex. Though there is a lot of overlap, each also showcases components that are not in the other.
Creating components and integrating components are two very distinct (though related) topics. Before deciding to get this book, you should know that it is definitely weighted more towards integrating existing components.