Friend is my answer. In the car, at home, and yes work-outs too. I can definitely survive without it during a ride or run, but sometimes that little bit extra can be given if your heart is pumping to some electro, hip-hop, top 40 bar star song, or really whatever gets you going. I've never raced with music, it can be dangerous, and well I like to hear the crowds, cars and all others around me in a competition. But during training (especially during the solitary work-outs), music is well; just peachy. Yesterday I had to coach a run clinic, so instead of my regular evening group ride, I did the work-out solo in the early afternoon. After 40 min of warm-up and pump-up to some random electro remixes I settled down with a repeat of Dj Khaled "all i do is win" for my 6x5 min steady repeats...(I got the song off of So You Think You Can Dance). First three were TOUGH! But I was able to pull them out all really consistently, the 2nd being the fastest. Number 4 is always hardest, in any type of repeat set for me (or the half way point). But once number 5 began I switched songs to something a little lighter and decided to let my body carry the music instead of the other way around. So I turned it to 'king without a crown' by Matisyahu (my favorite artist right now) and let the song and my effort get progressively faster. Number 6 was an absolute delight, replayed the song and churned up the hill again.
Only precaution when having the ipod in is; watch for cars! I only keep one head phone in when around a lot of cars, or I pause the tunes in a busy intersection. Train safe :)
-Emma

http://www.youtube.com/watch?v=eUgYG2bLuuw&feature=related