The Half-Blood Prince was partially ruined for me because I recently reread the sixth book and had extremely high expectations. The movie was light and funny in the beginning and I was disappointed with the lack of its similarity to the novel, but I understood the need to speed it up a little. There was a certain scene around christmas break that was stupid because it was not in the book and didn't add to the plot.
All in all, it was a good movie, but the plot could have been adapted better. A friend of mine had never seen any harry potter movies and he hated it. But when a Harry Potter fan doesn't have the book fresh in their mind, then they will enjoy the sixth movie.