I am reliant on mirrors when driving a car or truck and won't drive one until I have the mirrors set the way I like them. I never had any luck getting satisfactory results from a mirror on a motorcycle though. Shoulder checks worked much better. I am assuming that a bicycle will be the same.

If you look out the corner of your eyes in addition to swiveling your head, it doesn't take much head movement to get a quick look at the traffic behind you.