Harry Potter and the Half Blood Prince did not seem like it was as long as 2h33m due to its ability to keep you involved and interested with good use of visual effects and witty dialogue. However with the 7th book already out for a few years, and the knowledge that there are two more movies, the writers of the 6th installment forgot to include a substantial story-line and merely used JK Rowling's novel as a mere stepping-stone to the next big thing.
I would recommend this movie to anyone, But as someone who read all the books going to see the movie with someone who hadn't read any, I clearly enjoyed it more, and understood more of the subtleties. Good movie, not great, but definitely worth seeing again.