I personally don't use them because I need to hear all that's around me. Those damn earbuds don't stay in my ear when I am not doing any activity and I can see it being a pain while riding. As a matter of fact, I don't even use them, I have a nice set of headphones I use when I want to listen to music with my phone or iPod.
What I found interesting is one bike club encourages ear buds for route guidance, via bluetooth and RWGPS (they can get turn by turn guidance from their phones for the ride), yet encourages verbal communication on the ride (i.e. on your left, debris, turns, potholes etc.). Ummmm?
We did a ride last week and I saw several potential disasters and all of them were wearing ear buds. First, we are riding in 2 pacelines and this fool decides to come in between us and the guy with earphones on did not notice him and drifted right towards the idiot who then drifted towards me, fortunately I had already started backing off when the fool came between us. We tried to warn the guy with the earphones, but he could not hear us and they touched, but fortunately did not go down. Here's the kicker, the idiot that came between the 2 lines was upset because the guy drifted towards him. Damn fool. I was glad we dropped his ass. Second. Another biker riding with earbuds was all over the road and as a car approached, I yelled "car back" several times and he never heard me and continued swerving all over the road and the car had to just be patient to get around him. This is one reason some of the drivers get ****** with cyclists.
Another group I ride with is very adamant that no one is to wear earbuds during the group rides because as we ride the paceline, we need to be able to hear all verbal commands and warnings. That I like.
Another issue is when I ride trails, everyone has earbuds, but get upset when you wiz by them. I yell "on your left" but they don't hear, so when I see they have on earbuds, I just wiz by them. They don't hear squat.