Harry Potter and the half-blood prince is one of my least favourite HP novels. I've always found it maintains a bit of a holding pattern, particularly after all the action in Order of the phoenix. Lord Voldemort doesn't even make an appearance (except as Tom Riddle in copious flashbacks) -- after dominating HP5, which culminates with a climactic confrontation with Dumbledore. As a result, Half-blood prince seems anticlimactic to me.
The movie, though, could almost be the best of them so far. I say 'could' because I'm reserving judgement until I see it again. It's easy to be blown away the first time. However, it somehow manages to convey all the desperation of a world in despair that the book didn't for me. It is a very atmospheric movie, dark, lots of rapid movements, lots of sweeping camera shots with interesting angles. I really liked it cinematically.
There seems to be vast number of new characters as well. There's a real focus on the students in particular -- many of them new -- although once again there are very few classroom scenes. Somehow, the movie carries more weight. It's less 'cartoony'. A bit like the difference between Star Trek and Babylon 5. All this, despite a huge emphasis on teen romantic entanglements (not the strongest aspect of this movie).
The boy playing Tom Riddle is sufficiently creepy. And this would definitely be the strongest movie for Draco Malfoy. In the book he behaves mysteriously and Harry is obsessed with following him around and spying . . . which gets repetitive. In the movie we witness all of Draco's angst and conflict as he carries out the Dark Lord's commands. He does a great job and it helps add weight and tension to the movie.
I did feel a bit let down by the ending. I can't put my finger on it, but somehow it lacks sufficient weight. And there are always the things that get left out. More Lupin and Tonks! And although Ginny emerges a lot in this movie, I still don't think they bring her out enough. She's such a fabulous character in the books. She deserves more, particularly given her relationship with Harry.
But on the whole, I really enjoyed this movie and am very much looking forward to seeing it again. Maybe even in 3D!!