This is exactly the kind of thoughtful, useful technical writing I have come to expect from the "pragmatic bookshelf" series. Brian Marick's conversational writing style makes the subject immediately approachable: In just a few pages the first ruby/cocoa application is up and running, providing the reader a great incentive to dig deeper.
And deeper the book digs. I was expecting a good amount of Ruby and perhaps just enough Cocoa to be dangerous, however, the depth of Cocoa coverage this book provides is staggering for its size. I daresay that for the average programmer, this book could replace a lot of the unfriendly Cocoa documentation available from Apple. The author has clearly gone to great pains to research this subject and the reader benefits from that leg work, with many of the pitfalls and gotchas of Cocoa revealed before they become problematic.
The pace of the book is brisk, but I found it about right for the subject. As with all of the pragmatic books, the reader is spared from lengthy diatribes on language history, irrelevant trivia and interminable minutiae associated with some software books. I did not feel short-changed for length either; the book weighs in at ~370 pages plus an extremely useful glossary, which serves as a very handy desk quick-reference.
Unfortunately, RubyCocoa is already fast becoming a slightly obsolete technology, as the author acknowledges in the introduction. MacRuby will be upon us soon. However, given the depth of Cocoa information in this book I would still encourage any Ruby developer who intends to work seriously on desktop applications in RubyCocoa to absorb the basics from this book before hiding behind the MacRuby facade.