I saw the Dark Knight today at my local Imax theater. Of course it's brilliant. I have never seen a movie in Imax before, so I really didn't know what to expect. I had a brutal hangover when I went in, so the scenes where Batman decides to leap off a building about had me retching on my shoes.
Heath Ledger has completely justified the hype as the Joker. It really bums you out after you leave the theater that there is no more Joker to see. Ledger nails the acting, but the amazing achievment is how well the script nails the character. The Joker is sadistic and actually scary. They basically turned the Joker into a serial killer.
Go see this movie. It's amazing.