I am a huge Jack Nicholson fan, so was a bit disappointed whenever he wasn't cast in the role of the Joker (maybe he is getting a bit old for the part?), but Heath Ledger could not have done a better job. I never thought I would say I preferred another actor's portrayal of a character over Nicholson, but Ledger made a runaway performance.
The joker was totally a menace. The ending was a bit anticlimatic, but I smell another sequel -- Bring it on.