It's been a real pain to find a mirror (and bell) that will fit on the 31" "oversized" bars on my new Jamis Aurora Elite touring bike. But I just ordered this one:
http://www.rivbike.com/products/show...-mirror/20-177

I got a mirror about a year ago for my hybrid bike which I use for city riding. I find that it really helps me judge how much space a car is about to give when passing, and also to check for quick merges. Since I now have my touring bike I don't use the hybrid every day and don't take it on long rides, but still the mirror is useful.

Oddly, I also find that drivers yield to me much more often when I use the mirror (as opposed to turning my head), I guess because they don't think I'm looking.