I have the round mirror attached to my helmet and I like it okay. I don't love it.
The game changer for me though was getting a Garmin Varia tail light with radar. It works with my Garmin bike computer, or you can download an app for it on your smart phone. Alerts you to cars approaching from behind, shows how close they are getting, and shows you how many. It's the single best bike accessory I've ever bought.