When it's playing so loud that I can make it out, as if I were the one with the headphones on, then it's too loud, no matter what genre/band/song you may be listening to. Craps! Don't you value your hearing? Probably why it's turned up so loud, you're going deaf!
But if it's just loud enough that I know someone's listening to music... I'm a little more lenient, but still... save your ears.
EDIT: This is when I'm on the bus, and yes, when you're on the bus you'd like your music loud enough that you can hear it, but you shouldn't make it so that it's completely drowning out the bus noise.