My commute is on a main road with continuous +45mph traffic. I ride dropbars and it's difficult to get a good look behind you while also maneuvering through road debris and watching for cars racing out of driveways between breaks in the traffic. Then there are the intersections where a cyclist going straight has to cross the right turning lane.

I got the 'take a look' eyeglass mounted mirror two years ago and it's added to my safety. With a quick tilt of the head I see everything to my left and behind me.