While there are certain things I would have liked to have seen added to the film, overall the tone, atmosphere, and attention to detail in the back stories and sub-plots is the best its ever been in the series.
The characters are much more fully fleshed out, allowing for a much deeper look into the hearts and minds of previously unexplored characters.
Alan Rickman plays Snape to a T, and receives quite a bit more face time than in previous films.
Outstanding special effects work, though these days such things are to be expected.
Exceptional movie, would definitely recommend for all Harry Potter fans. Half Blood Prince is a welcome addition to the world's most successful movie franchise.