This book is designed around preparing the reader for passing a Sun certification exam, namely the "Web Component Developer" exam. As a result its emphasis is on a thorough conceptual and terminological grounding. It's a pretty good book on that level, but it's not a good reference once you've learned the subject, and it's not a good book for working through practical examples.
If you're familiar with the Head First series you know that the style is both very visual and informal. You may find this a little off-putting if you're looking for a "serious" treatment, the tone is definitely light, while its presentation is often cartoony. But it is very accessible and its approach for introducing the topics seems to me well thought out. If you don't mind the occasionally strained attempts to be funny I think you'll be able to appreciate its pedagogical drift.
I found it a valuable introduction, especially the earlier chapters. Chapter 3 was the best of the book, and taught me more of what I needed to know about Sakai development (when I knew very little) than anything else I've yet encountered. Highly recommended for the novice web application developer, but otherwise probably not so much.