I've always been very anti headphones - bit silly when you need to be hearing things etc.
But - recently have occasionally popped in my IEMs and chucked my iPhone on shuffle for my commute home, as its all bike path and no cars to deal with. Turns out to be quite enjoyable, especially after a shit day at work.
Am yet to experiment on the trails, but will do the next solo ride. Most times though I enjoy solo rides for the quite - turn the world off for a while and just enjoy the moment.
I use Nuforce in ear monitors, I like their tonal qualities - especially when fed lossless files. In ears are the way to go, got into them when I commuted by train in Melbourne and they block out the idiots nicely! These days I use them at work mostly, block out the office when I'm trying to get some work done...