In the Harry Potter saga, Harry Potter and the Half-Blood Prince is a key entry. The first three books introduced us to the characters while the fourth and fifth books were more set-up as they laid the building blocks for what was coming. It was the sixth book that upped the ante and kickstarted the climax as Harry finally learned how to vanquish Voldemort and began to prepare himself for it. The film, which expertly mixes romance, drama and action(though not in equal proportions) manages to convey this key aspect and is another solid entry in the series.
Considering the length of the book, it is understandable that some segments have been excised for the movie. Still, the choice of things that have been left out is somewhat surprising. Some interesting twists have been altogether removed while other sequences have been replaced with somewhat weaker ones.
The film's trajectory remains the same as in the other films. The portions that deal with the social side of the lives of Harry, Ron and Hermione translate better to screen. The actors have grown with their roles and our familiarity with them works in the movie's favor. Ron's snarky comments and Hermione's know-it-all responses are rather muted but they do make us smile several times. Most of these smiles come from their romantic feelings and entanglements as they have trouble expressing themselves, do so rather tentatively and experience pangs of jealousy. But we also feel for them when they realize that their feelings aren't reciprocated.
The dramatic parts are weaker compared to the book but its no fault of the actors or the screenplay though. Partly because we know what's coming and partly because there's no time for what's happening to sink in, these portions don't evoke the kind of exhilarated reaction the book did. This happens whether the developments are surprising(as when Harry finds out about the Horcrux) or emotional(as the happenings at the end). The action sequences again don't have this problem and the short Quidditch trials session and the sequence in the cave are superbly picturized.
The book left us waiting breathlessly to see how it all ends. The movie allows us to relive the book and has once again made us wait in anticipation to see the end. That makes it an unqualified success.