Harry Potter and the Half-Blood Prince has strongly broken the streak of mediocre films that was only interrupted by Alfonso Cuaron's lyrical take on the third book, Harry Potter and the Prisoner of Azkaban.
David Yates has clearly learned from his previous gig, and his predecessors. He's figured out how to play with the space that the Hogwarts School of Wizardry and Witchcraft provides him and J.K. Rowling; he probes the shadows of the old castle to uncover those lying within the characters. The links between the students, their teachers, and their school have always been the movies' greatest assets. Bellatrix Lestrange's destruction of the Great Hall at the end of the film, followed by her torching of Hagrid's cabin, are not mere desecrations of spaces, but deaths of characters, important as any other after six films.
Playing the fated-to-die Albus Dumbledore, Michael Gambon is at his highest and his lowest in this film, and those opposing moments come within seconds of each other when he's with Harry in the cave. As he's drinking the poisoned water, begging Harry to kill him, and when he sets the world on fire, saving Harry's life. His silent communication with Snape, just before he's killed and falls off the tower, is a beautiful rendering of an essential moment in the book.
Really, though, the Harry Potter film series' lasting contribution to the world will have everything to do with its acting. Not simply because it reminds an impatient society the virtues of patience (seeing the young stars grow into skilled, even powerful actors), but because it's introduced a generation of young film-goers to a who's who of great British actors. The supporting cast members have been, without variation, extraordinary. Jim Broadbent, as Professor Horace Slughorn, surpasses the bulk of those that have come before him. He's haunted by Voldemort from the instant he appears onscreen up until the moment he admits to Harry that much of Voldemort's mad rise to power is directly attributable to something he once foolishly mentioned without thinking.
The moments where the movie most clearly diverges from the book – the ones that feel most directly inspired by the tone of Prisoner of Azkaban – were certainly among the most compelling. Harry's final moment among the Muggles, where he flirts with a cute waitress who knows far more about the art of seduction than he, and the Death Eaters' attack on the Weasley family's home at the Burrow, bringing home the danger of Voldemort's rise to power in a very Children of Men-esque way (from the completely-not-in-series-character shaky cam, to the instantaneous thievery of the power the characters felt only a moment earlier, to the senselessness of tragedy). These were the moments where the movie became its own entity, separate from the novel.
For a movie as dark as this one was, there were a lot of laugh moments. It balanced the brightness and the shadows quite well. A wonderous experience for an audience invested in the series thus far.