Whenever you see a movie that's based on a book -- especially if said movie is based on one of your most favorite books -- you almost have to detach yourself from that book in order to enjoy the movie.
I feel that I did just that successfully with the motion picture adaptation of Harry Potter and the Half Blood Prince. The result? I love love love love loovvvvvved it (It kinda annoys me when people do that – it's not something I do lightly; yeah, it was that good).
If you haven't seen it yet, then don't read the comments because that's where I'll be posting my thoughts.