I use a tiger eye mirror clipped to my helmet on the left side. It took some time to adapt, but I preferred not having to break posture to do a quick sweep behind me when I wanted to make a left in a neighborhood or check traffic behind me to plan for passes by other road users. The only problem I had was with positioning the mirror. At first, I couldn't point it behind me and keep it in my peripheral vision, so I bent the arm until I could. Then I noticed that even after that I couldn't see it sometimes, but that was because my sunglasses frames blocked it or because I had my helmet cocked back on my head. In my opinion it's indispensable for riding on roads so I can plan for the faster traffic around me, and it doesn't vibrate like the handlebar mounted mirrors tended to.
When I ride singletrack I leave it on so I can glance at my group (usually my kids) and see how they're keeping up.