I use a round bar end mirror on my flat bar commuter. I commute through fairly mild traffic with only a few busy places, so the mirror is handy for knowing when a car sneaks up behind me. I also frequently turn my head about so I am not surprised by something I didn't see in the mirror.

I don't have a mirror on my road bike. I do use the road bike occasionally on my 27 mile r/t commute, and find I don't really miss the mirror too much.