I saw this yesterday and I was completely blown away. But I think people who go to watch this movie expecting a fun movie are going to be disappointed. It's anything but fun. It's intense and unsettling, at times almost too much so, especially the scenes with the Joker.
Yes, Heath Ledger's performance as the Joker is something that needs to be seen to be believed. The Joker is a villain with no rules or even any sense of purpose in his actions, he revels in chaos just for the fun of it. He kills people wantonly and derives pleasure from it. Due to the total unpredictability of his actions, Batman struggles to stop him, and as the body count goes up, resorts to very dubious means to try to get to him.
That was actually the best part of the movie for me. Without giving away too much, it was how Batman is tempted to break his own code of justice, of what is right and wrong, when the end seem to justify the means. Two scenes highlight this point, when out of sheer desperation he does things that certainly one does not expect of him, things that seem to be at odds with what he believes in. But it is totally understandable, it's what any man would do when faced with losing everything that is dearest to him.
He is after all, unlike other superheros, only human.