Just got back from seeing it a couple hours ago, and all I can say is: Wow.
Of course Ledger's portrayal of the maniacally genius Joker was nothing short of outstanding, but I think a big "kudos!" has to go out to the main cast all around. Morgan Freeman, Michael Cane, and Gary Oldman really stepped up, and I felt that Aaron Eckhart put in an especially fine performance as Harvey Dent.
This movie did almost everything right: The action scenes & cinematography was top-notch, the dialog well-written & acted, and the characters were brilliant. This isn't just an adrenaline-fueled action flick, it's a story involving sharp moral choices/conflicts and genuine human pathos. (Even the Joker manages to elicit a certain macabre sympathy, although not so much in the "poor guy" sort of way.)
Oh, and major props to Nolan for having the courage to kill off a major character -- did *not* see that coming!
Not since V for Vendetta
have I watched a film that managed to blend heart-pumping action with an excellent human morality tale so seamlessly. This is easily one of the best movies I've seen in the last several years, perhaps ever.