You've already heard the overhype, so let me just say that it was wicked cool to see so many shots of Chicago, my hometown until last year. I'd forgotten that Dark Knight was filmed there, and it was especially nice to see bits and pieces of the city--Chicago as I'm used to seeing it in real life--instead of the usual distant skyline.
Oh, and something else that the reviewers aren't mentioning: the Joker's arpeggio is the finest since the shark's in Jaws. It's that powerful.