This is absolutely the "Empire Strikes Back" of the Harry Potter series. It's not a movie, as much as it is a bridge; a link between what has happended up to this point and where we know it will end -- an incomplete piece.
The viewer is left mostly satisfied, but craving to see how this will all end. If you haven't see any other Potter film, you really wouldn't know what is going on.
Potter fans -- go see it. New to the Potter Universe -- rent the other movies first (or read the books), then go.