I've been reading it recently out of curiosity, since I've heard it mentioned a lot in passing. The primer on javascript is as clear a basic reference as I've ever seen, and his ideas are pretty sound.
I think the JSON vs XML debate could have been touched upon a bit more, but he doesn't seem that keen on JSON for readability reasons (personally I'm in the pro-JSON camp) so maybe he skimped a bit on that area. Perhaps some simple database driven examples might have been a nice addition as well - I felt there were about 3 really solid chapters that would be beneficial to anyone new to AJAX, and the rest would be more useful for people who are relatively novice programmers who are new to just about everything.
The last chapter about the future of AJAX seemed a bit of an afterthought, and didn't really have anything useful in it.
Overall though, as a primer on the subject it did a pretty good job, and would recommend it for anyone who is new to AJAX, even if you're not that familiar with Javascript or the DOM. Experienced programmers might not get so much out of it.