I ride with my ipod. It's loud enough to hear but I can still hear my bike and the same things I can hear without the earphones in.
I think riding with music being a good idea or not depends on who you are. Some people get "pumped up" or whatever with their music on which can create careless riding habits and a big distraction for them... I'm not so much that way. I'll wear them while riding the mountain too. I notice that when I do this though and I turn the wick up a bit, I don't even hear the music anymore. Then when I back off a bit, I'm like... oh yeah I have my ipod on. Almost forgot.