Watched Harry Potter and Half Blood Prince last week and I thought it's not that interesting as compared to the other series. I've never read the book, so I can't really comment on difference between the book and the movie. But from the movie, it feels like there's no climax. When the movie ended, it was like "is that all?" even though the movie itself was quite long.
Feels like this movie is to prepare us for the ending of the whole story and no matter what, we got to watch it in order to watch the last of the series the next time round.