Overall, this is an excellent book for learning Cocoa. I might go so far as to say this is the best introductory book for a new framework I've ever read.
Cocoa, like Rails or Django has the unfortunate consequence of having to teach a language and a framework all in one. If you're diving into Cocoa, you'll probably have to learn Objective-C, C and Cocoa. Coming from someone with hardly any C experience whatsoever (I knew the basic syntax, and the basic idea of how pointers & references work), I was able to pick up this book and get going fairly easy.
That's not to say there aren't some rough patches in this book. Chapters 26 & 31 stick out to me. The rest of the book feels very practical, but these chapters feel like they are dealing with some really advanced and abstract topics (color math & CoreGraphics respectively) while trying to teach basic topics (formatters & garbage collection). The end result is getting caught up in some abstract technology that's way over your head, and completely unrelated to the subject matter you should be learning.
The book took me about a week and a half to complete, digging at a couple hours a night in my free time. By the end of the book, I feel at least comfortable enough to try and build my own applications. I don't have all the answers, but I have a good idea of how to find them. And I think that's the key to any starter's book.