I decided to be kind and not call this post "Harry Potter and the Half-Baked Prince." That might have been too strong.
I don't know many people who have not read the book and not read spoilers, but if you don't want the surprises to be ruined, you can turn back now.
You see, I think that book 6 of the Harry Potter series was not as good when stacked up against its predecessors. I think that several parts were okay, but overall the book was quite anticlimactic. :-/
The problem with Harry Potter is not its writing, it is its success. Success does weird things to authors. For Rowling, in my opinion, success has decreased my enjoyment of her storytelling. For some reason, the series seems to have lost something because it's no longer relegated to the back shelves of a musty bookstore. Yeah, I know, I'm odd. :D
One good thing about The Half-Blood Prince is that Harry Potter and friends are finally growing up. Rowling promised us this in prior books, but for me, it has come too slowly. The characterisation in the book is vivid and great, that is something that I enjoyed through and through. It is the plot and resolution therein that I found lacking.
The book kinds of ruin the unity of the series. It really throws a lot of things out of the window. Firstly, it introduces a new plot twist via the "Horcruxes" of Voldemort. Good idea. Tad too late. Now when I review the series in my mind, book six is this sort of huge bump in the road that sticks out rather unpleasantly. In other words, too much happened too fast.
It is also quite anticlimactic. This is partly not Rowling's fault. The penultimate book to a series, especially when it is the second book of a trilogy, usually suffers from the lack of resolution. The first book works because it leaves the reader wanting more, and the third of course resolves everything. But the second book is just a step along the way, with neither a proper beginning nor end. Book six suffers from this, but Rowling can only do her best to temper the effect; it's very hard to stop it.
Overall the book was enjoyable and caused a lot of pathos (especially Dumbledore's death) but I found its unity to be lacking. Good characterisation, but the distribution of action was uneven enough to make the book a little difficult to read.
But I find comfort in the fact that the books remain better than the movies.