If you are riding on the road with automobile traffic I highly recommend a rear view mirror. Being able to see what it is coming up behind you while keeping an eye on what is in front of you is critical.
Below are some rear view mirrors I've tried. The last one in the list is the one I use.
These are fixed to the handlebar ends. It is hard to site-align with traffic.
This mirror is fixed to your wrist it is hard to site-align with traffic.
The mirror on this one was too small for my liking. Wasn't thrilled with the Velcro attachment either.
This one attaches to your eyeglasses. Seemed too close to my eye, blocking my forward vision. There are times when I don't wear eyeglasses. Like when they are covered in sweat and dirt.
This is the one I've used for several years. It attaches to your helmet with cable zip ties. The mirror is large enough for a good view of traffic. It takes a while to align so that it doesn't block your forward view.
I've replaced mine a couple of times over the years when the mirror looses its reflective qualities due to water, dirt, and abrasions. It is way over priced but considering it lasts a year or more and it is vital to my safety it is worth it.
Side view of my helmet
I added highway department grade reflective tape on the back of the mirror and all over my helmet.