Played it for myself, completed the campaign and majority of side missions.
This game is amazing, and an example of a game where it does justice on it's predecessor.
The ending is actually quite sad, if you look at it from the Joker's point of view too. His death was simple but sweet, and the look of his character made it seem like by the time he died, his body reflected the madness within. I loved how he etched a smile on his face as he died too.
It's a pity, but I hope it's not strictly true, because I'd love another Arkham game with the joker in it.