One night me a few mates got massively drunk and for a laugh decided to turn aim assist off and play the game. Well the next day I had forgotten that we had turned aim assist off, I only realised 3 weeks later when I went through my options. I turned aim assist back on and didn't really find much difference.
I don't think it helps a great deal. Maybe just centres on an opponent quicker but gets confused when you are shooting at an opponent from a distance and an opponent closer runs past, it switches to the closer opponent sometimes which can result in your death but not always.