The target audience for Harry Potter and the Half-Blood Prince already knows what happens in the movie. An interesting phenomenon with a movie like this is how it works to enthrall that audience and still speak to the rogues who haven't read all seven books.
As a member of the latter group, I can say it's easy to appreciate the level of production put into realizing these images, which should in turn satisfy the former.
Harry Potter and the Half-Blood Prince picks up Harry (Daniel Radcliffe) living on his own in London. A slight dose of real life—hitting on a waitress, reading the newspaper—grounds the next two and a half hours in our world so that we can appreciate the magnitude of the magic to come. When Dumbledore (Michael Gambon) whisks Harry off for a magical quest, the nearly grown-up Harry acts nonchalant, because this is his world. He's lived it most his life, so it's real to him.
With a new potions teacher (Jim Broadbent) at Hogwarts, Harry signs up for class and discovers an old spell book that belonged to the Half-Blood Prince. The Prince's notes and shortcuts help Harry excel in class, and in some of his personal endeavors. Meanwhile, he still has to help Dumbledore uncover secrets of Voldemort's past.
The spell work and visual spectacle of magic artifacts make the evolving world of Hogwarts look more grown-up. It's not quite the fantasy look of the first two films, when perhaps the visual effects were in their freshman year, too. Between the growth of the technology and the aesthetic of the later films, the magic now looks like something tangible, not just painted on the frame by digital artists.
The strongest aspect of the sixth film is the exploration of teenage dating with a magic twist. Harry and Ron (Rupert Grint) start asking out female classmates, leaving out Hermione (Emma Watson). When teenagers can use spells to express their heartache, that elevates the evergreen issues out of John Hughes territory.
The main plot works itself out rather mechanically with surprises and double crosses, but since the majority of the audience knows the story already, this shouldn't be a problem. It's the nature of long-winded novel-style storytelling, but everyone likes it, so who's to complain by part six? Something about the inevitability makes Alan Rickman's delivery of Snape's lines awesome. Getting there is a visual thrill, with emotional treats along the way.