Years ago I put one on my left handle bar grip and bore the brunt of the cycle purists jeers. Now if I rent a bike somewhere I constantly look for it. I find rear view mirrors very useful,especially in traffic or when measuring with a partner.
I also tried the helmet and eyeglass mounted rear view mirrors. These I didn't like, too hard to quickly find the mirror in my field of vision.
You should have heard the jeers when I added a kickstand. I wouldn't be without that, either.