My main reservation about the book is that not much actually happened in it. I generally get the feeling that this will act more as part of the series than as a standalone story. That said, there was a decent amount of character development, which is good. I also think that I need to give it a second reading, to re-assess certain scenes in a different light (now that I know how it turns out), but I'll wait a little while before I do that.
The big scene in this story (did I mention those major spoilers?) is of course when Snape kills Dumbledore at the end. This shocked me at the time, but now that I've had some time to think about it I'm pretty sure that this wasn't actually a betrayal, i.e. that Dumbledore wanted him to do it.
Going back to the start of the book, Snape already knew about Draco's task before Narcissa/Bellatrix came to see him, so it's plausible that he'd told Dumbledore about it. For that matter, we don't know for certain how Snape found out about it (since we didn't "see" Voldemort tell him), so his information may actually have come from the Order of the Phoenix. Anyway, I think that Dumbledore told Snape to do whatever was necessary to gain Voldemort's confidence, including committing himself to the Unbreakable Vow, in the same way that he later told Harry to follow his orders to the letter, even if that order was "leave me to die and save yourself". For that matter, even if Dumbledore didn't know about the plan at that stage (if Snape was just trying to stall for time to continue spying, never intending to carry out the task), he may have later decided that it was necessary to go along with it - the fact is that Snape was saving himself by killing Dumbledore, since otherwise he would have died (by breaking the vow).
This is related to a couple of other scenes in the book. At one point (in the forest?), Harry overhears Snape and Dumbledore arguing, where Snape says something like "I know I agreed to do what you wanted, but I've changed my mind" (I'll dig out a more precise reference for this when I re-read the book). So, was he rebelling against his undercover assignment (as Harry thought), or was he saying "No, I refuse to kill you!" I'm now leaning towards the latter. Then, at the end of the book (at the top of the tower), why did Dumbledore freeze Harry? The only reason I can think of is that he didn't want Harry to intervene.
As for the actual killing, it's almost "unreliable narrator" syndrome, since we're seeing things so much from Harry's perspective. But there is certainly an alternative way to interpret what we're shown. Dumbledore pleads with Snape, saying "Severus ... please ...", but he doesn't say "Please don't kill me". At the same time, Snape has "revulsion and hatred etched in the harsh lines of his face" - it's implied that these are his feelings toward Dumbledore, but I think that they were his feelings towards the task he was facing, i.e. the murder, and Dumbledore was actually saying "Please do kill me", although he couldn't say that explicitly because it would make the other Death Eaters a tad suspicious.
Looking at Snape in general, I've felt for a long time that he's the most interesting character (although that's not to say that he's my favourite character). This goes back to the end of book 1, where the conversation between Harry and Dumbledore goes something like this:
D: "Snape saved your life"
H: "I thought he hated me?"
D: "Oh yes, he does, but he saved you anyway"
And as I recall from book 3, Snape was annoyed that he'd never been able to repay James Potter for saving his life (in the Screaming Shack). I think that explains his return to Dumbledore's side, after he'd told Voldemort about the prophecy - not that he was at all fond of James/Lily, but that their deaths had put him into a bad situation.
Throughout the series so far, Dumbledore has kept saying "trust Snape" (with gradually decreasing levels of patience), and he hasn't been wrong yet. This ties in to Phineas' comments in book 5 (Phineas being the former headmaster and Sirius' ancestor), and the fact that Sirius would still be alive (or at least needn't have died then) if Harry had trusted Snape. So, I'm inclined to go along with Dumbledore this time. That said, he was overly optimistic about the Occlumency lessons (in book 5), since Snape couldn't overcome his personal hatred of Harry enough to do his duty to the Order.
Actually, now that I've written all this, it seems increasingly obvious that my theory is correct (rather than being a brilliant flash of insight), so I apologise if I'm boring you all with what you already know.
On the wider subject of "the bad guys", I felt a bit sorry for Draco, although he's still not exactly a good person, and I do wonder whether Myrtle could have acted as mediator between him and Harry. I also think that it's about time Hermione reversed the jinx on Cho's friend, rather than leaving her scarred for life - it's served its purpose (by telling the DA who betrayed them), and there's no more damage she can do to them. And it's nice to see a decent Slytherin (Slughorn) - he's true to the spirit of the house, by being interested in power, but he's not partisan about it.
Moving into speculation about book 7, baratron has suggested that Dumbledore is going to come back, having used a Horcrux to preserve his soul. I'm rather dubious about that, since Slughorn said that you have to commit an act of ultimate evil to split your soul in half in the first place - if this does turn out to the case, then I think Dumbledore's sordid past as an axe murderer would be more significant than his actual return. However, he may well go down the ghost route instead, or simply show up as a painting, to join the "council of former headmasters" on the walls. I'm still a bit hazy on how that magic works - would his painting have his knowledge from the time of his death, or just from the time that the picture was painted? I'm guessing the latter (or similar), since that would allow McGonagall to confer with him, without him revealing the specifics of his plot to her (since he presumably didn't want her to know about it in advance).
As for the rest of the book, I was impressed that we're still getting variety out of Quidditch games, rather than them all blurring together, and Harry's promotion definitely made it more interesting. I also thought that Luna's commentary on the Gryffindor-Hufflepuff game was very funny - the dialogue between her and McGonagall ("I can't remember his name, it's something like Bibble - no, Buggins -" "It's Cadwallader!") really made me laugh.
On the romantic side, I liked the Harry/Ginny pairing, but I'm less sure about Ron/Hermione (I think it's a natural development for the characters, but it probably won't work out in the long run). I was also interested in Hagrid/Maxime - I hadn't really appreciated before that they'd be in a long distance relationship, since they're teaching at schools in different countries.
As a general thought on the series so far, there are various "blog-image quizzes" that ask questions like "Which Hogwarts character are you?" I've rarely found them much good, since they tend to turn into "Pick the answers that match your favourite character", or ask impossible questions ("Are you better at Charms or Transfiguration?"). And the simple answer is that I'm not any of them - I'm me. That said, I think the character I'm most similar to is Neville. As with my comments on Snape above, this doesn't mean that he's my favourite character (I'm not sure whether I actually have one), but I was glad to see him doing well in this book.
I've also been pondering about the world building aspects of the series, specifically the educational system. It doesn't look as if there's any equivalent of university, although some careers offer their own training (three years for Aurors). But how about primary school? If not, haven't wizards outside Hogsmeade already grown up among Muggles? I'd think that this would give them a better insight into things like telephones. As a related thought, I'm guessing that Hermione is going to become a teacher, since she talked about doing something worthwhile with her life, and she has the brains for it.
Anyway, roll on book 7. And in the meantime, I'm guessing that it's about time for another "Torg Potter" parody at Sluggy...