I am done. 652 pages and I am done in less than 24 hours after I got it. I am damn impressed with myself, I must say. Back "in the day," when I was a teen and wasn't allowed to watch TV, I once read "2010: Odyssey Two" in less than 4 hours, which was 354 pages, IIRC. I think my total reading time was about 8 hours, which is still in stride, compared to how complex it was to follow everything and actually comprehend what I was reading. To a dyslexic, this is always a victory. It wasn't speed reading for speed's sake, but it was reading with comprehension. When I re-read book 5, for instance, I was delighted that even though it had been 2 years since I had last read it, I remembered pretty much everything important. I did this earlier in the month to prepare for book 6.
Half Blood Prince is a good book. Lot of surprises, some nagging questions finally answered, and some of "I KNEW it!" You can completely tell Rowling's growth as an author. This is barely teen material. The word "slut" was used, along with "snogging," which is about as vulgar as the word "shag" in British terms, even though snogging is a lot more PG-rated than shagging. A lot of "what makes someone evil" and "what makes someone loyal" is discussed. This is a thinker's book.