And as far as I can tell the auto aim feature is disabled or not there at all in the demo because there were plenty of shots that missed by fractions of an inch that would have automatically hit in other games.
Tumbler, Auto Aim was on by default in the Demo. I turned it off when I first played through (having just recently come into possession of a 360 myself and departing from PC-only FPS playing, I totally suck with the controller... but 'AutoAim' still sounds like a dirty word to me)...
At any rate, towards the end, I was getting my ass kicked because I couldn't hit anything with my Plasmids in the sudden flurry of attackers; I played back through the 2nd time without disabling auto-aim. Trust me, it was *much* easier to hit everything with it on.