Seemingly a better movie to sit through than Order of the Phoenix, but just like that one, the earlier moments of the movie were better than the latter ones. As is the trend these days, this movie has a dark tone, and is the final movie to chronicle Harry's daily life in Hogwarts. That said, it falls into the same trap a lot of book adaptations does, shortening and omitting events. This has the effect of things not fleshing out correctly, not explaining things thoroughly enough such that it confuses viewers (if they didn't already know what was going to happen, i.e. read the book) and making it harder to enjoy as everything is rushed. A lot of focus was put towards the relationships between the protagonists, sure, I guess they need to make it more noticeable rather than just coming out of nowhere, but it seemed out of place and forced. I would have preferred if they hadn't had changed the way Harry got the memory out of Slughorn, it was perfect in the book. The way Harry does in the book (already having learnt that he was asking about Horcruxes) seems more clever and puts more emphasis on repeating the same actions as Tom Riddle. The cave section, where Dumbledore retrieves the next Horcrux wasn't done good. They managed to drag on and on about the relationships but when it came to the proper plot elements, they do a rushed job.
Many revelations were not explained and even the depiction of the small island seemed weird. Finally, what happened to the climax of the film? It's weird to note that in the book there is a major battle at the end, while in the movie, possibly due to time limits (and if so, then it's quite stupid considering they can cut out a fair few boring/useless scenes), there is no fighting. So, they managed to add in a stupid pointless battle at the Burrow, but couldn't add in a small fight in Hogwarts that was actually canon? *spoiler* Dumbledore's death and Snape's betrayal was disappointing and doesn't deliver the emotional impact that it should. *end spoiler* Then there's the title, the importance of the Half-Blood Prince was just shoved aside, and when it came to actually finding out who it was... you don't care, coz this "Prince" (or his textbook, to be more precise), didn't play a large enough role to have any effect on what had happened in the film. While it's easier to sit through and there were some good humor, I would rather just spend the extra couple of hours sitting through the book, at least then, you would fully appreciate the intricacies of the plot.