I use them quite a lot on my PC. If I'm on mic to somebody then it goes without saying that I wear them, but I'll wear them anyway since the sound quality is a lot better through headphones, plus I just like that it drowns out 'background noise'.
I'll occasionally do it with a handheld since the speakers on them are total dog crap compared to any headset I've got hanging around, but I'm usually not near a headset when I use a portable so it's not that often. So many times I've found a soundtrack from a handheld game that had a groovy bassline or something I didn't hear before because the audio hardware is arse.