Aim Assist is an option in the "Game Options" that helps players to aim by slightly locking on to enemies. When a player positions the crosshair over an enemy the Aim Assist helps keep it in place by automatically adjusting the aim as the player and enemy move, commonly known as "Sticky Aim".
This is often shunned by snipers, as when coming in and out of scope, Aim Assist will auto-target the dead centre of your opponent, making it troublesome to re-aim for a critical hit. On the other hand others may argue that the auto aim is especially useful in close quarters as it can be used to 'quick scope' an enemy. Additionally, a consistent sniper would learn how to be able to use this ability to its full potential resulting in critical hit 100% of time.
Aim Assist can be particularly problematic when targeting Crawmerax's minions from the WNW glitch point. Aiming at any minions which have spawned close to Crawmerax can result in the Aim Assist targeting Crawmerax instead, making it extremely difficult to maintain an accurate aim. Turning off Aim Assist solves this problem.
Aim assist is mostly only helpful when fighting a lone enemy who doesn't move around much. Adjacent targets moving across each other can pull the aim away from the intended target. Aim assist is especially frustrating when trying to blow up a vehicle with a rocket launcher from far away with a large scope, because again the scoped in speed is too slow to catch up to and pass the vehicle in order to lead it enough to get a bullet on it.