That is to say deliciously dark and disturbing. Within the first five minutes - less than that even - you're sucked in and then the rest of the time battered around as the plot gets darker and darker. Very bitter and slightly sweet.
Why is it so good? Because this film is not just about the characters of Gotham City. It's only through the characters that many questions are asked and some answers are made about humans as individuals and civilization or society as a whole.
One side comment I would make is that parents should NOT bring their children to this film. After two and a half hours of suspense/horror/action/drama I was completely dazed and confused to the real world around me. It's just a very heavy film that no child should see - for the child's sake.