i often turn music off ingame because i'm tired of hearing the same god damn music over and over again. If more maps had custom music, i may consider listening to it.
as long as the song isn't part of the map, aka doesn't play when you have music volume to 0, i'm fine either way, but when someone makes a map that has music on it and you have music playing in winamp behind ut and you hear both songs it ubersucks and i hate the map automatically, even if the map is cool...
A 3 minute song repeated 10 times over the course of a 30min game would do my head in. Nice, quiet ambience looped would suit me as long as it's not too intrusive on the SFX.
I like it when people use good rave or techno music in their maps, like DB-Nightstorm had. That is a wicked song. As long as the song is good I want it in.