I found two reasons to like the Half-Blood Prince film and two to dislike it. To consider the good things first, the movie is a joy to look at. I could watch it again just to stare at individual scenes, ignoring the storyline while marveling at the seamless combination of camerawork, set design and digital effects. The classical pace allows one the luxury of absorbing some detail before frames are whisked away.
One's enjoyment is enhanced by some fine acting. The cream of British actors has appeared in this series, though not all of them performed as admirably as one would have hoped (Kenneth Branagh and his ex-wife Emma Thomson were notable disappointments. Helena Bonham Carter isn't great, but I believe she was over-rated in her early days anyway). In Half-Blood Prince, Jim Broadbent makes a brilliant addition to the cast as Professor Slughorn, Michael Gambon finally puts his stamp on the role of Professor Dumbledore and Hero Fiennes-Tiffin makes a sinister young Tom Riddle.
The downers? They involve the adaptation from book to screen. There's far too much time devoted to developing romances; those bits weren't much fun in the books and get really tedious in the film. The second peculiar decision is to leave out the final battle. The reasoning, apparently, is that the Battle of Hogwarts in the final episode would then seem like a repetition. That's an unbelievably stupid way of thinking. The fight at the end of HBP is tiny in scale compared to the final battle. It is important because it offers some tiny release after Dumbledore's death.
I've said I liked the film's pacing, but it desperately needed some action at the end.