Music or no music all depends on the environment to me. If I'm running on a beautiful trail and can hear birds chirping and the trees rustling in the wind to me nothing is better I wouldn't think of covering up those sounds. However, if I'm on a treadmill in the gym at 5:30 pm on a random Wednesday then I wouldn't leave home without my i-pod fully charged with lots of random music all with thumping high tempo rhythm to keep me going.
It's been years since I did my cardo at the gym, so walking outside, I don't use music, even in the city. I do need to look for more interesting walking routes though. I went to the beach last week, and it was great.
If I hear a great rock song before I swim, sometimes I keep that song in my head and it really helps me stay focused, excited, and to push myself as much as I feel I want to.
I like listening to upbeat dance and Techno style music for my cardio and Hip Hop/Rap for my strength training. For some reason that really gets me pumped up.