It is clear in this book that Harry is definitely getting older. The main issue here is that we get to see more of the adults in the story; in particular, they interact with each other independently of Harry. I've been thinking recently about "the innocence of youth", and I think that it's not so much innocence vs guilt as it is a matter of awareness - young children tend to be fairly self-centred. It's similar to Tolkien's stories - when I read "The Hobbit", I sympathised with Bilbo, since Gandalf had sent him off in his quest, then basically abandoned him. When I then read "Lord of the Rings", it turned out that Gandalf did have other responsibilities to attend to, i.e. a life of his own that didn't revolve around Bilbo. There are similar themes that appear here.
To use a phrase from "Babylon 5", this is the year that everything changed. It was certainly interesting to read all the new developments. Following the trend of the previous books, the subject matter is getting darker, and some parts of it were genuinely unpleasant. However, there are funny bits spread around the book, to keep it from being too depressing.
One thing that did strike me about this book is that it's clearly aimed at people who have read the other four, or at least seen the films, which I think is a safe assumption. While there are a couple of brief recaps to jog your memory, there's no standard introduction. So, for instance, the book refers to Muggles without explaining what the term means.
Another impressive achievement is that I read about (yet another) Quidditch game, but it still kept my attention. I think it's quite a challenge to write about so many games, and yet keep them distinct.
So, all in all, a great book - well worth the money. And given the amount of publicity surrounding the launch, I'd recommend reading it sooner rather than later, to maximise your chances of avoiding spoilers. Oh, and incidentally, I definitely like the fact that the hardbacks are approximately the same height as the paperbacks, rather than being oversized - this makes it much easier to shelve them.