First and for most i would like to say that I enjoyed the book. However, Im now depressed (just like after watching Revenge of the Sith) that i have to wait so many months, maybe years, for the next and last installment.
Before i start off on my criticisms i would like to again warn those who have not read the book that this entry has MAJOR SPOilERS.
I finished the book in around 7-8 hours over the course of a 1.5 days. "HBP" was great but i didn't enjoy it as much as "Goblet of Fire," or "Order of the Phoenix." However, Rowling didnt tell us much more detail in this installment except for the background of Voldemort. The book did put together bits and pieces from previous books in the series, including that Tom Riddles diary in "Chamber of Secrets" was a Horcrux that Voldemort put some of his soul into it to become immortal, but for the most part the novel gave us more questions then it answered.
One of the first things you notice in"The Half Blood Prince" is exactly what has been lacking or, for use of a better statement, what it doesnt have enough of is the adolescent romance that usually goes on in a school setting. Of course you can tell in "Goblet of Fire" there was some hormones raging and obvious relationships in "Order of the Phoenix" but this book has a huge subplot of "who is going out with who." Rowling has done a good job leading the reader into wishing that two characters would eventually get together as "Friends" did with Ross and Rachel. So in book 7 i will be looking forward to if Ron and Hermione become a couple. But the romance in the novel was a double edged sword. The romance felt that it was there in place of the usually enigmas that plagued Harry, Ron, and Hermione, thus making this novel in question of a definite plot.
The fact that Half-Blood Prince turned out to be Snape wasnt a huge surprise to me as in the other books. When the Prince was revealed it was more of an "of course" then an "OMG, REALLY." No, instead the huge surprise became the death of Dumbledore, at the wand of Snape. Rowling said that someone died in this book but i didnt think that it would be a major character as Dumbledore, in fact I thought that I would be Percy Weasley, Cornelus Fudge, or Hagrid who perished in the story.
A few things I like about the story as not to lead you to believe that i hated the whole novel is that it has raised so many questions of whats going to happen in the final story. I have a couple huge revelations on whats going to happen in book 7. One thing is that Dumbledore's death was real but planned. Yes i believe that Snape and Dumbledore made their own Unbreakable Vow, and Dumbledore left many hints and clues to help Harry find the remaining Horcurxes. I have other speculations but ill save that for another entry.
"HBP" does set the base of what needs to be answered in the last novel. I just hope that it will be a last book as worthy as the phenomenon that is Harry Potter.