Maybe it's just because I use the Safe Zone mirror that has a big field of view, but mine kind of sits at the edge of my vision so I'm basically always getting input from it. If I know I want to change lanes and traffic isn't thick, two solid glances into it for maybe a second each, separated by a second or two, confirm that there isn't a car coming up behind me. I actually don't look over my shoulder very often anymore. Only once have I definitely had it save me from being run over, a large delivery truck was driving in the bike lane behind me and I was able to swerve over into a bus stop before he hit me. But the reason I wouldn't ride without one is how much easier it has made lane changes for me, especially multiple lane changes, like to get over to a left hand turn lane.