So far I have worked through chapters 1 up to 21 (out of 38 chapters) of the book. As an after-hours iOS app developer and aspiring Mac OS X app developer I found this book really easy to follow and I feel that I'm really learning a lot during each chapter. I love the hands-on approach that is followed in this book. Wherever you need to do things in Interface Builder, there are more than enough screen shots to guide you and make sure that you don't dwell down the wrong path. Indeed, I'm pleasantly surprised that so far I didn't get stuck building the examples even once! After guiding you through an example, the book explains a bit more about what is going on under the hood in non-excessive, but sufficient detail. I highly recommend this book to any software developer who would like to get started with Mac OS X development!
Side note: as the book mentions, knowledge/experience about Objective-C 2.0 is a requirement - the book recommends reading the book The Objective-C Language [...].