But, you need to be at the right stage... which is basically, when you can follow the book, although it may take a bit of effort. If you're already a ninja, you probably don't need to read the book at all, unless you simply want to be exposed to another POV. And, for a lot of non-ninja, the book will be too advanced. Luckily, O'Reilly put the entire first chapter on-line so you can judge for yourself.
Chapter 1 is no namby pampy intro. In the chapter McCaw defines a constructor function used to create constructor functions that emulate classes in languages which support classes natively. He also includes a useful discussion of how the 'this' context switches in JS and how to control it with bind or by defining a jQ-like proxy method. Later design patterns have some similarity to what he does in chapter 1 (using Object.create instead of constructors), so if you can follow this chapter, you're probably ready to take on the book.
Some of the other reviews have touched on a few negatives, but to my mind they're not enough to downgrade the book. Occasionaly, the discussion seems to jump over an explanatory detail, but if you make a lab page that links to the book errata page and download the code for the examples, you should be able to fill in any gaps. I found the first five chapters fascinating, and chapters 6-13 useful and concise roll ups on various topics like dependency management, debugging and various libraries. In addition there are appendices that do a quick survey of jQuery and CSS3.