Bike mirror use and non use

Whether I'm cycling on a highway shoulder, in urban traffic, or on a group training ride, I find a cycling mirror to be indepensable. When I'm on a group ride I'm surprised by the few riders that use them.

Before totally bashing the idea of wearing one, see if you can try one first. you may have to borrow a friends.

Ok now, let's hear your pros and cons. I'm curious and eager to respond.