Meh. Harry Potter was never exceptionally original, but this is feeding cannibalistically on the still-twitching carcass of the other HP books, and not in a clever or surprising way. The plot moves forward in a satisfying way, but the storytelling is not there to back it up, and character development seems to happen between pages or between chapters.
The darkness is satisfying—Chamber of Secrets, which is still, in my opinion, the darkest of the series, and the best, remains my favorite—, but the characters don’t really seem to have shifted with the tone of the text, and the result is an odd juxtaposition of adolescent angst and war drama; the sort of thing that makes sense in the campy atmosphere of, say, Buffy the Vampire Slayer, but has more trouble getting off the ground in the more fanciful, naive, and un-ironic world of Potter.
As with many Harry Potter books, this makes me wish I had access to Susan Cooper’s The Dark is Rising (my copies are unavailable or destroyed), of which Harry Potter can be seen as a vastly inferior copy. I seem to recall reading in Zelazny somewhere (Isle of the Dead, I believe) that in James Bond (et al), sex and violence are proxies for love and death; I think that Harry Potter has some of the same issues (though obviously manifesting somewhat differently); and Cooper’s grasp of love and loss and her ability to present it through the experience of children is far more impressive than Rowling’s. Fantasy, especially British fantasy, has long been a breeding-ground for morals, but one must deal well in subtlety, complexity, and nuance; Cooper, L’Engle, White, Tolkien, and Lewis all have this quality (in varying degrees—Lewis, for example, tends toward the obvious at times); Rowling does not, at least not yet, though it might be interesting to see where she is at in ten years, if she can ever recover from having written Harry Potter.
That said, it was reasonably entertaining, and, as I said, the plot moved forward—and these things are not necessarily guaranteed; witness the lack of plot in book four and the look of entertainment in book three.