I finished up Harry Potter and the Half-Blood Prince last night. If nothing else, Dame Rowling kept me turning the pages, I must say. I also have to say this was probably the most predictable of the Potter books. I did everything I could (to successfully) avoid coming across any spoilers. I don't think it was the best of the series, but better than Order of the Phoenix and still good enough to keep me turning the pages rather quickly. The scenes with Harry and Dumbledore were both good and annoying. Good in that more was revealed about Voldemort's past, but bad in that Rowling repeated, through Dumbledore, what she just showed us as occurring, in the vein of "Well Harry of course you know what we just witnessed was how bad Voldemoret is and why he is that way," but a bit more lengthy.
I liked the way the relationship between Harry and Dumbledore grew in this book. Dumbledore's character came more fully into view through their relationship, and Harry grew too. On the whole, the book could have been trimmed in some spots, but I am really looking forward to how Rowling ties up the series, answers many of the questions left by the Half-Blood Prince, and reveals if any of the clues she has been leaving were red herrings.