Depends on what kind of mood I'm in, but I don't like Auto Aim. Sure, I'll use it if I HAVE to, but if there's an option to turn it off I'd gladly turn it off.
If the game uses autoaim well I have no probs with it, but some games pretty much stop you from getting headshots and some games follow the person/AI with the reticle, which can get in the way if Im waiting for a head to go past, and the reticle moves to compensate... grrr.