I know there are folks who have their reservations about JKRowling's series. Some dislike Rowling's writing style and are eager to point out her perceived instances of adverb abuse and other stylistic lapses that disqualify the books as "literature" in their eyes. Some dislike the book on religious grounds and have been demonstrative of their distaste for the witchcraft element to the books.
The recent dust-up about Pope Benedict's "supposed" disapproval of the books have more to do with discrediting the Pope than cheering Harry in my opinion. (I think his "letter" reads more like the kind of boiler-plate note that gets mailed out when unsolicited books arrive from authors hoping for a notable comment from the rich and famous. Check any of the vanity press websites to see similar letters from presidents and celebrities who are acknowledging these "gifts" as kindly as they can.)
The OpinionJournal has a very thoughtful review which expresses many of my own feelings about Harry Potter and the Half Blood Prince.
There is maturity of another kind in this book that sets it apart from the rest of the series thus far, and that is the depth of its moral message. This may comfort readers made uneasy by claims that the Harry Potter books are somehow satanic or by Pope Benedict XVI's recently publicized criticism of the series as potentially harmful to children's religious formation.
For gone is the implied but relatively crude Manichaeism of the earlier books. The struggle between Good and Evil is enriched, this time, by explicit talk of free will, the power of love and the sanctity of the soul. In one scene, a teacher at the Hogwarts School explains to a student: "You must understand that the soul is supposed to remain intact and whole. Splitting it is an act of violation, it is against nature."
"But how do you do it?"
"By an act of evil--the supreme act of evil. By committing murder. Killing rips the soul apart."
At another point, Professor Dumbledore explains to Harry that he is not compelled by either destiny or prophecy to pursue Voldemort, who murdered Harry's parents and countless other innocents. The boy has free will and can choose his course of action.
"It was, [Harry] thought, the difference between being dragged into the arena to face a battle to the death and walking into the arena with your head held high. Some people, perhaps, would say that there was little to choose between the two ways, but Dumbledore knew--and so do I, thought Harry, with a rush of fierce pride, and so did my parents--that there was all the difference in the world."
Thanks to Petrified Truth for the tip