I play on Assisted Aim servers.
The weird thing is that my friends and I reported that we started out as Free Aim on online and then changed without us noticing. We've been too lazy to change it back.
This is the part that bugs me more... I don't use free aim online because I can't find a server with anyone else when I try... and if you join a friend, he'll likely be using AA. I've never seen a Traditional GTA aim - and when I experimented with it offline, I found it wasn't very traditional to gta... (how could it be with all the controls so different?)
I used free-aim through the whole SP campaign and would prefer it... the AA always locks on to the guy who's NOT shooting at me (against the AI) so I was much more effective w/o it.
Again... nothing about the combat in this game requires any skill.. either AA or Traditional GTA AA or free-aim.. it's all on super duper easy mode with mega kill laser beams...
If I had the option to change it in the pre-game lobby, I would change it more often (as host). I do think it would make a decent feature add (allowing us, the host, to choose the aim mode for the game we're about to play). That way.. say you choose a Deathmatch, you can choose to go Free Aim (which most people would agree would require more skill than AA) but if you go on a mission you can use AA (and no one will care because AI's are super-human auto aiming nightmares).