I didn't want a mirror. They are so not cool. They are for old people, and after all, I am (cough cough) the new 30.
But the organizers of the Seattle to Portland strongly recommended one, so I bought a "Take-a-Look" glasses mounted mirror.
I wore it a day or two before the ride and didn't like it much.
Then I wore it during the ride.
It was flippin' great. Surrounded by other riders, riding in an unfamiliar area, while trying to make sure I didn't ditch my sweet wife riding partner was it bit unsettling. The mirror helped a ton. I got used to just glancing up in no time.
When I got home and took a ride without it I kept glancing up for it, and it wasn't there. I guess it takes 200 mikes to adjust, but once I did I really miss it when it isn't there.
I guess I'm a Fred...