My rating: 3 of 5 stars
As I may have said in one of my earlier reviews, each of these books can be captured with one apt adjective. Unfortunately, the adjective for this book is "predictable."
The major argument against my assessment will be the fact that this book is "necessary to set up HP #7." However, I think this was a shortcoming on Rowling's part. After all, she wrote all of the other books as stand-alone, complete stories in their own right. I see no reason why this book should have been any different.
Which, of course, leads me to my disappointment with the book's predictability. One of my favorite features of the HP books has been their unpredictable endings; Rowlings always writes her novels in such a way that no reader, no matter how closely they follow the details of the story, can fully predict the conclusions. Being a close reader myself, this gave me something to look forward to in each book, because making predictions--and being delighted when they are turned upside down--is one of my favorite things about reading. The Half-Blood Prince, however, came out exactly as I expected. Of course Draco was bad, but not bad enough to actually kill Dumbledore. Of course Dumbledore had to die. Of course the Horocrux had to be a fake, after Harry and Dumbledore went through all that heartache to get it.
The novel was by no means boring, but ultimately, it left me disappointed. Then again, however, Rowlings certainly obliged her readers to follow through the end of the series, because if they didn't read #7, they'd never find out of Snape had really betrayed Dumbledore or not....