I finished this yesterday evening, but haven't had a chance to write about it until know.
This is, of course, Book 6 in the Harry Potter series. I assume pretty much everyone has already either read this book or already decided they don't want to read the series, but just in case I have a reader who just arrived from Mars, I should explain that this is definitely part of an ongoing story, and you should start at the beginning; the first book (Harry Potter and the Sorceror's Stone, or in the UK, the un-cheesified version Harry Potter and the Philosopher's Stone) is a little bit weak, but it's quite short, and the second book (HP and the Chamber of Secrets) is a good representative sample.
This book does show that the minor distraction of being made more wealthy than Queen Elizabeth by the success of the earlier installments have not completely destroyed Rowling's ability to write. The characters still seem like the the same characters, the world is still engaging, and the plot advances significantly to the conclusion we're all expecting in the seventh and final book. I don't want to include spoilers, so I'll just say that a major ongoing story thread took a decisive turn I hadn't really expected, major things happened to characters that I didn't really expect, but it seems to be fitting together and heading for a more coherent conclusion to the series than I really expected. Through the first five books, I wasn't really convinced that Rowling really knew exactly where she was going with the series, but book six makes me think that she really did have the whole thing planned from the start. I'm definitely waiting to read the seventh book; we sort of know that the good guys have to win, but it will be interesting to see just how it works out, and also to see if Harry goes ahead with the mistakes he seems resolved to make at the end of this book.
Harry Potter isn't as good as the hype (what could be?), but it's fun. And this one, despite having things that bothered me, is a solid chunk of the story. 8 out of 10.