I keep my gamma so that everything looks nice and atmospheric. Only maps I can't see on are Bast and Lost. I am pretty horrible at playing Lost, and I get lost constantly, and generally fall in that big whole with the RT (obs bridge?). Bast... needs some fixing, and usually when I play there I just crush myself with an elevator or something to end the pain.
On a more serious note, I think its better to have things a little darker, and just hope that the enemy is using similar settings. ^^