Caveat: I am only basing my review on the sample chapter on Performance Tuning, a draft copy of the chapter on Zend Engine Internals, and the table of contents. Also, I may be biased because I've met George and he's really hard not to like--I'll try harder next time.
Go read the reviews of other PHP authors' blogs by doing a Feedster search of this book, you'll find that they highly recommend this book, sometimes over their own books!
Looking at the chapter list, you can see the topics covered are not what you see in a typical PHP book. In fact, if you don't know PHP, this is definitely not the book to be learning out of. If you don't know how to program, this is not the book to be learning out of. But if you use PHP professionally and need to get that extra mile or scale your PHP to a large site, then this is definitely the book to be buying.
There are simply no books, articles, or web references that provide a bird's eye view of the Zend Engine Internals. That is, until this book. The car analogy in this chapter is very apt for the potential reader of this book on a whole. If all you do is use PHP to get you from "point A to B" you certainly don't need this book. But if you are a PHP professional who is responsible for a high performance website, then you need to know the content of this book cold.
The chapter on Performance Tuning has a lot of gems you can't find in any other PHP reference and is essential knowledge when you use PHP on large sites. According to Michael Radwin, George's APD (the tool referenced in the chapter), is used to performance tune parts of Yahoo. But George also gives a nod to competing projects mentioning their strengths and weaknesses. (You can see why it's hard not to like the guy, his pragmatism supersedes his ego in every page.)
This neatly complements any other PHP book on the shelf (including any other "professional" PHP book). And I'll definitely have to give an updated review after I've read the other chapters.
I'm sure it will pay for itself many times over. Believe me, I speak from experience to know that George speaks from experience (he was involved in a PHP-based company that drove a company I worked for out of business).
(And to all of you who have received advanced copies... Grr!)