First off, be advised that if you're running Snow Leopard you likely won't be able to use this book. Automatic Reference Counting is used throughout the book and only works on Lion. However, if you come from a background with Mac or iOS programming and have knowledge of retain cycles, you should be able to fill in the blanks.
The books available relating to the latest and greatest version of Cocoa are limited, and this one seemed to be at the top of the pack, so I took the plunge.
The first indication of concern came from chapter one with the statement, "...assure yourself that you are not stupid and that some things are just hard."
No, nothing is hard if it's explained completely, and from the beginning. Which is exactly where this book fails. While it does start from the beginning, it doesn't explain things completely.
Up to about chapter 8, the author does a halfway decent job of explaining things, but leaves out many of the important little details. For example, the author rarely ever discusses the properties in interface builder for various controls and objects. Only the specific ones that are used are discussed, albeit in very brief detail. To make matters worse, interface builder in Cocoa apps are often complex and it's very easy to make mistakes for someone without previous Interface Builder experience. These mistakes will certainly leave users frustrated at a minimum and require them to recreate entire projects at worst.
Around Chapters 8 through 10, and especially in chapter 9, the book appears to make a change from bearable explanations to almost none at all. Granted NSUndoManager is an involved topic, but isn't that all the more reason to add more information to the Chapter? Apparently not, because it's only 12 pages long with half of them being "type in this code." Even at the end of the chapter the book says, "We don't really expect you to understand every line of that code now..." I guess they don't expect you to EVER understand it it because they don't cover it anywhere else in the book. As important as including undo operations are in an app, I would think this would be one of the thicker chapters.
In chapter 10, the book goes from limited explanations to almost none at all. There are huge code blocks added that the reader needs to type, with explanations virtually non-existent. Simply type the code in, and figure it out on your own.
After chapter 12, the book goes back to the more tolerable explanations. In fact, at points it becomes quite good. This is a shame, really, because I feel that many aspiring Mac developers will be put off by the difficulties the previous chapters will give them.
As an iOS developer, I was able to get by with this book, but for those unfamiliar with iOS or previous Mac programming experience this book will be VERY daunting.
If you still decide to buy this book, I would recommend the Apress book "Learn Cocoa on the Mac" to go along with it. It's dated, but it fills in many of the blanks as to how and why things work.