Let's start off by saying what this book isn't; it is not a 'how to learn Objective-C' or 'getting started with Cocoa' but something, in my mind, equally useful: how to put what you've seen into the context of writing an application.
The book isn't just a rehash of others, it adds a lot of useful things that seem to be skipped over in other books: setting up localisation files, project settings (version numbering), error handling, help files, apple script support, etc... so you really do get an understanding of 'how to approach' coding within the environment and see alternate ways of doing things.
Okay, now for the things I found difficult:
1. You don't get to write the whole application: that pop's up about 40% way through the book (see the introduction to recipe 7) - this is a real shame a I would have liked to have gone through all of the UI elements
2. It can be hard work when there are few diagrams in between the pages of detailed descriptions. I am sure that the amount of text, in the book, could have been significantly reduced with some good diagrams, which would have made it easier to digest (for me)
3. You DO need to down load the code files as the book does contain errors (I've reported the ones I've found back to the publishers) and some of the instructions, in the text, did leave me wonder where I should be adding code...
Saying that, the book is still worth it, very worth it, and I know that I will be constantly using it as a reference alongs side Marcus Zara's 'Core Data', Daniel Steingburg's 'Cocoa Programming' and, of course, Arron Hellegass 'Cocoa Programming of Mac OSX'