It depends on the game but more and more these days I tend to play with it on all the time or whenever available.
WoW looks HORRIBLE without AA and beautiful with it lots of other games just look ugly without it too. Half-Life 2 for example looks pretty ugly and angular without AA or AF, lovely with it. Lots of "big" development titles now expect you to use AA and look horrible without it. In days gone past the artists would have had to worry about this and spend time making it look better now the tech is there they don't bother and it shows if you don't use AA.
Basically unless you want to have to spend a lot more time with your art assets smoothing everything off nicely it makes sense to use AA.
Darkheart
WoW looks HORRIBLE without AA and beautiful with it lots of other games just look ugly without it too. Half-Life 2 for example looks pretty ugly and angular without AA or AF, lovely with it. Lots of "big" development titles now expect you to use AA and look horrible without it. In days gone past the artists would have had to worry about this and spend time making it look better now the tech is there they don't bother and it shows if you don't use AA.
Basically unless you want to have to spend a lot more time with your art assets smoothing everything off nicely it makes sense to use AA.
Darkheart