(Warning: Spoilers! If you haven't read the book yet and don't want to know what happens, stop reading now...)
So I finished the book on the flight home from Washington. Actually, I wasn't quite done when we landed at LAX, so I had to grab a seat at the arrival gate and finish the last few pages. Rowling has turned out another great page-turner, with delicious prose throughout. (She has a great ear for language, and the books are wonderful to read aloud.) Now of course we have to wait probably another couple years for the grand finale.
As to my predictions, I was wrong about some, though gladly so. I'm glad that Ginny wasn't using love potion after all, but was instead just being a teenage girl, and Harry a teenage boy. (Nice "Spiderman moment" there at the end.) I'm also glad that Tonks was herself and is back to herself, though I was carrying my polyjuice impostor theory up to the end. (When Tonks showed up in the hallway at Hogwarts, I began to think that Malfoy was polyjuicing into Tonks.)
My big hunch was correct, although I certainly wasn't expecting the way that Dumbledore meets his end. However, even though Snape is looking pretty dark, I'm standing by my assertion (which will remain unproven until the final book) that Snape will ultimately come down against Voldemort. Dumbledore will not have been wrong about trusting Snape, and in fact in the end I think he wanted Snape to kill him. It was clear from Dumbledore's conversation with Harry before their last quest that he was fully prepared to be killed. And given the way he was slowly slumping down the wall at the very end, I'm wondering whether the potion he drank wasn't going to kill him anyway, and Snape was just putting him out of his misery. Notice that even though he had every opportunity, Snape managed to completely avoid harming Harry. While he reminded the other Death-Eaters that Voldemort's orders were to not kill Harry, it certainly would have been Death-Eater-like to bruise Harry up a bit, but Snape did no such thing, and even prevented the others from harming him. Also remember that Snape could have easily killed Dumbledore earlier if he had wanted to, when Dumbledore fried his hand on the first horcrux. Instead, Snape (as Dumbledore himself reports) saved his life.
Indeed, I think redemption will be a major theme of the final book. Snape is a triple agent, but he's lost the only person on the good side who really trusted him. Somehow, Harry will have to learn to forgive Snape. In the previous book, Harry had got a glimpse of Snape as a schoolboy being cruelly teased by Harry's father and his friends, and had a pang of sympathy for the guy. And Harry developed some attachment to the Half-Blood Prince before realizing who he was. Somehow Harry will need to overcome his hatred of Snape. (Will we get an explanation from Dumbledore himself, whose ghost now lives in the portrait gallery in the headmaster's office?) The other remarkable thing is the seeds of sympathy being sown for Draco Malfoy. We get a glimpse of a frightened young boy alone in the bathroom crying about a horrible task he feels extorted to perform, and then we learn that his motivation is to save his mother's life. Draco loves his mother, and his mother loves him, and both will do anything to save the other. That's not exactly the makings of a good Death-Eater there, and Voldemort ought to know that (just as the totalitarian government in Orwell's 1984 knew that it could not allow two people to love each other), but as the prophecy suggests, that just may be his ultimate blind spot. Look for some Death-Eater defections to play a role in Voldemort's undoing. As Harry has grown in the last two books by learning that the guys in the white hats aren't always perfectly good nor perfectly invincible, in the next book he will learn that the guys in the black hats aren't always perfectly evil. You heard it here first.