On the plus side, I do believe that Half-Blood Prince is the best written of the series. Prisoner of Azkaban is my favorite book plot-wise, but Half-Blood Prince is definitely my favorite language-wise. It's so well done, and the first two chapters are awesome. A comment from J.K. Rowling's website on the opening chapter, "The Other Minister" . . .
"I have come close to using a chapter very like this in 'Philosopher's Stone' (it was one of the discarded first chapters), 'Prisoner of Azkaban' and 'Order of the Phoenix' but here, finally, it works, so it's staying. And that's all I'm going to say, but when you read it just know that it's been about thirteen years in the brewing."