So fantastic. I won't say anything because I don't want to contribute to any spoilers, but the book is absolutely incredible. Like the series finale of Six Feet Under, this final book made reading all of the others more worthwhile than I could have imagined.
At some point I need to go back and look at how Rowling plotted out the novels. From a craft perspective, she did exactly the right things and I think writers of fiction can learn much from her techniques.
If you haven't read the Potter series, do yourself a favor and read them. They're not only for children or only for people who read "those fantasy books." They're good storytelling, plain and simple.