Edit: Oh yeah, spoilers!
OMFGNUUUUU! ;_;
Does not compute, Snape was bad all along? But, no, I understand he took the Vow, but once again Rowling leaves so much more to guesswork. Even though the first two chapters break away from the normal, giving us unusual insight into what goes on outside Harry's circle. But really, that just raised even more questions than it provided answers. Tricky woman, she is, oh yes!
I was reading it very carefully, and had guessed or supposed who the Half-Blood Prince was and what Malfoy's orders were before very early on. This didn't take away any enjoyment though, because I knew there would still be surprises, and there were. I think it was better than Order of the Phoenix, mostly because Harry wasn't doing as much obnoxious screaming in OMG ALL CAPS!!! Just as good I think, as Prisoner of Azkaban.
That's really all I've got to say, still digesting as I've just finished a few minutes ago.