I love the Half Blood Prince movie. It does introduce some of the worst continuity problems of the movie series, as it's among the most different from the novel, but I think it manages to become one of the best movies of the bunch. It's one of my favorites to watch, as it reminds me why I actually LIKE the characters in the book. I like that it actually managed to be consistently funny and entertaining.
The only lasting complaint I have about the movie is that they rather ruined the entire Ginny storyline, but I think that's partly casting. Bonnie Wright is lovely, but her portrayal of Ginny has almost none of the spirit that character brings to the book. The moment when she yells at the Quidditch team is one of the most unbelievable scenes in the movie - her control there feel completely unearned, and I'm not sure why Harry likes her version of Ginny. I do wish they had included more of the Snape backstory and Voldemort flashbacks, as it's going to be remarkably difficult to ret-con all of the Lily materials, but I believe they can find a way to reach the same conclusions. And honestly, after the thoroughly overstuffed and completely unsatisfying mess that is the fourth movie (and the first movie, for that matter) I'm willing to let them make plot-cutting decisions in exchange for a more enjoyable movie-going experience.