First off, I'm a huge Bat-fan.
This particular title I'd put off reading for years, but finally read today...the whole in one sitting (because it isn't broken into chapters and, well, it was pretty that good, too!)
Anyone who knows Azarello's work (like 100 Bullets) knows that he can tell a great story in the spirit of, say the Godfather or Goodfellas and Joker is no exception.
This is a Batman story, in a very loose sense, but primarily it is just a good story. It doesn't rely on the Bat-world and could just as easily have been adapted to fit Spiderman or Daredevil (one their darker villains, anyway!).
Anyways, as a good story, I think it's fantastic. As a Batman story...no thanks.