I liked the non-technology specific sections. Browser tools included Safari and Opera plugins. All the "hot" security topics were covered (SQL injection, XSS, CSRF.) Tradeoffs were listed for different alternatives. Performance included CPU, memory and bandwidth. Trying out examples on the companion website was nice.
Many server side techniques were PHP specific, such as SQL injection. While six pages of code is good for PHP developers, I was surprised. The code was readable without being fluent in PHP, but unnecessarily narrows the audience. The 15 pages of screenshots/description on the PHP documentation tool could have been used for another topic. Wouldn't a PHP developer already know how to use PHP?
While I liked the presence of an accessibility chapter, I was confused. WCAG/Section 508 were introduced clearly early in the chapter. For the rest of the chapter, I wasn't clear on what pertained to WCAG, what was 508 compliant and what was coming in the future.
If you are a hands on PHP AJAX developer interested in architecture, this is an excellent book. Non-PHP developers or people who want to focus on architecture (rather than code) are better off with a different book.