To give a totally un-spoilered general comment, anyone who still says Harry Potter is for kids hasn't read the book. While the first book was aimed at 8-10 year olds, the vocabulary of #5 is way beyond primary school. I'd imagine most children under about 13 or 14 would have to read the book with an adult or a dictionary to get the best enjoyment of it. Also, the story has grown up tremendously - this one is multi-layered and dark. #4 was a bit disappointing for me, because I thought it was good but over-long - there were lots of places where the story dragged. #5 is a significantly longer book, but well-paced all the way through.
I really like the character development in the book. Most of the characters are allowed to mature. I'm particularly pleased that Ginny is allowed to get over her pre-teen crush on Harry. A lot of fanfic features 17 or 18 year old Ginny still fancying Harry as if she was 10, and frankly, that's not how adolescence happens. The tension between Hermione and Ron continues realistically, and the Harry/Cho romance develops in the way relationships at that age tend to.
The new main character is gloriously evil. I like. I also like that both the Dursleys and Snape are made more human. Before, they were almost cariacatures. Now, there is some rationale given for their actions. I found myself almost feeling sorry for Snape.
In general, a lot of things that have already happened are explained in the book. I'm pleased that my idea for why Dumbledore sent Harry to live with them is the actual reason given. Also, that nothing has happened which seriously affects the sixth form and post-Hogwarts fanfic that's been written. Even The Death is a character we could afford to lose. And it's foreshadowed enough to not come to us as a serious unpleasant surprise.
Will write more detailed oohing and aahing when everyone who wants to read it has. Now I have to get on with my work.