Finally went 1000/1000 earlier today. Really terrific game. So far I've only done 1000/1000 on two 360 games. This and Dead Space. And both are going to be annual playthroughs.
Arkham Asylum is really the perfect length with phenomenal pacing. I enjoyed everything about it, even the boss encounters. Sure, some of them bordered on too easy, even on Hard, but truthfully...I prefer it that way. It makes the entire package pretty well balanced.
For nail-bitingly hard-to-the-point-of-frustrating gaming, you play the challenge modes. Going for high scores on those is a fucking workout.
But when you just want a nice, comfortable experience, you play the story mode, where the obstacles are challenging but not aggravating.
I was doing the challenge maps on Normal with a second save file for Hard story mode, and it really worked out well. When I had enough of the combat maps, I'd play story mode for a bit. There was always something I felt like doing in this game, which isn't something I can say for most titles.