I just picked up a copy of this book a few days ago, and I was impressed with the in-depth sections on the book (XBL, RDF & XUL Templates, for example). I feel that the book is layed out in a very good way - the first section has overview of 'the basics' (CSS, XML, XUL), the second section has advanced features, and the third section had case studies. For people who are new to programming, and don't know XML, I would recommend another book before you start this one. The later chapters may be too complex and hard to digest for non-programmers.
For me, I am familiar with XML and some of the Mozilla technologies, so I could skip over the intro section of the book. I am finding a lot of benefit out of the XUL Programmer's Reference section, as well as the advanced section. This book, used in combination with xulplanet.com, can be a good resource.