The earlier movies are too reverential towards the books- and move too fast (at the expense of pacing and texture) because they feel bound to cram in as much of the plot and incidental detail as they possibly can. Half-Blood Prince doesn't make these mistakes. It takes a strimmer to what is possibly the weakest book in the series, foregrounds a handful of its huge cast, makes good use of Jim Broadbent, handles the theme of burgeonng sexuality better than Rowling does, gives Ginny Weasely something interesting to do and establishes a glooming atmosphere of suspense. Considered simply as a piece of movie-making, it's the best so far. It doesn't overdo the CGI. And if you narrowed your eyes and overlooked the silly costumes you might even think you were watching a straightforward (or not so straightforward) romantic thriller in the vein of Hitchcock's Rebecca.