Just finished THBP. I might have stayed up all night reading it, save for the sinus headache I've been sporting since our house became so much cooler and drier (the AC giveth, and the AC taketh away). Not sure; one gets used to the Harry Potter experience after a few books, and the thrill of discovery, of page-turning, is never quite as strong after 5 or 6 as it was after 1 or 2. Nonetheless, six books into a series she's not doing so badly; at least it was planned that way. (Don't get me started about Robert Jordan - since I have nothing good to say about his writing, I'll say nothing at all.)
The only comment I'll make, to join the non-spoilerish crowd, is that I wasn't as surprised as some appeared to be. Perhaps the non-spoilerish comments of others forewarned me, rather like the "OMG You'll never guess the twist!" made the twist in The Sixth Sense as obvious as an elephant in the fridge after the first shot of the cellar doorknob. (There's another over-rated person for you, M Knight, but I won't go there either.)
Aside from that, the series is built - like most fantasy, like all coming of age writing - on archetypes. Things progress in a certain order, and certain things must happen to the hero to create the growth required by the whole coming of age thing. In that light, neither the identity of the Half-Blood Prince nor the other shocker were much of a shocker - I'll say no more however since I'm hovering dangerously close to spoilerism.
I'm not saying I didn't enjoy it. It was pretty good, certainly a damn sight better than the bloated 5th book which preceded it. Shame on authors who allow publishing houses to convince them that an extra 30,000 words really will delight the readers, and that a bloated toad of a book is, in fact, just charmingly packed with more Potterish goodness. (I'm perfectly certain I'd fall into that trap too, in the highly unlikely event that I ever wrote something not only published, but popular.) Good enough for me to stay up half the night despite a fairly strong headache; good enough to make me want to read #7 as soon as I closed the last page; good enough to make me post about it. If I'd thought it was awful, I'd have held my tongue - I have no wish to be lynched by Parry Otter fans around the LJ-world.
But shocking, surprising, twisty? Not to me. Satisfying? Actually, yes. The requirements of the genre are being entertaininly fulfilled. Those who died, had to. Those who betreayed (or did they?), also had to. The Snape mystery is, to me, the best thing in the series, and that I'm still guessing after six books is a tribute to both good plotting and, of course, Mr Rickman's nasty-yet-human portrayal in the filums.
Odd, actually, how books and movies are blending for this series. They've done relatively well by me - when I read the books, I see the people in the movies, even though they don't necessarily match the descriptions word for word. I'm not sure I like Mr Gambon as Dumblydore, though I do like Mr Gambon... but the beard seems glued on when I see him, sadly.
I'm now rambling more than usual. Time to stop.