Before all the safety first people come on and say "that's illegal" but really in most all 50 states in the US its legal and I don't see it being any different than listening the a loud car stereo with or without thumping bass or people with headsets talking on the phone. To me the real danger is people texting but that's a whole different story. I wear mine a lot and I find that I concentrate more and look in my mirrors more often then when I don't have them on (which is rare) because I know I have to be more alert for emergency vehicles and everything. I don't have a really good car stereo though mine isn't horrible but with my phone or other pmp and a good pair of IEM's just make the drive that much more enjoyable for me so let me know if you wear your IEM's while driving and be honest about it.
For more about the legality of wearing IEM's / headphones checkout this link, http://lifehacker.com/5902802/ever-wonder-if-its-actually-legal-to-wear-headphones-when-youre-driving