To get the most out of the book the reader must follow its instructions, do the exercises and commit to learning the material in the way Head Rush Ajax teaches it. It took me a while to get used to the teaching style as it felt painfully slow and discontinuous at first. I played along, though, warming to it a bit, but ultimately feeling like I'd be better served by a less chaotic writing style, something more in the 'reference' side of the spectrum.
What I liked most was getting to play with JavaScript. I'm a server-side developer, mostly, so I enjoyed the book's coverage of client-side code: using JavaScript to make asynchronous requests, manipulate the look of the page with the DOM and work with the server's response data (XML or JASON). I was a little surprised to find that the term AJAX refers to a small subset of what client side developers have been doing with JavaScript for a long time.
Head Rush Ajax has whetted my appetite for more fun with JavaScript. It was a bit slow for my taste; I'd like a JavaScript reference rather than a book with classroom-like exercises. The format will probably work well for some, though. The authors work hard to anticipate reader questions and use a variety of methods to help commit the answers to memory. Head Rush Ajax is a good fit for beginning programmers. It's a light intro to JavaScript, the DOM and server response data in XML and JSON format.