I have mine generally 75%... Sometimes up to 85%...
I use it to drown out noise and listen to something good.
You can think of it this way: Loud enough so people say "Holy shit your music is really loud!" (I only listen with one ear, so I need it really loud to hear everything as much as I want to) But since it's only in one ear, I can hear the conversations going on around me. But when I'm in class I tend to turn it down towards 25%.
But when I'm really annoyed I listen to my metal on the bus with both earphones in and I turn it down to around 50%.
My music-listening habits are complicated.
[small]And I think everyone should hear the glory that is metal. (No, it's not all screaming an guitars. If that's what you think it is, you need a lesson in the metal subgenres.)[/small]