I tried some different kinds of bar-mounted mirrors with varying degrees of success. Some of them worked pretty well, some of them didn't. Then I tried a "Take-A-Look" mirror mounted to my sunglasses and have never looked back. Well, actually now I constantly look back.

It took a few rides to get used to it, but now I really find it useful. It didn't replace me turning my head and physically looking, but it gives me an additional option. It's especially useful when merging to the left through traffic (i.e. moving across lanes into a left turn lane) where I want to keep an eye on cars both in front of and behind me at the same time.

I still occasionally ride without it, like when I'm wearing goggles instead of my regular glasses. But I prefer having it there.

If someone else doesn't like them, that's OK too.