I use one - because in the time it takes to turn your head to look behind some idiot ahead of you can swerve out of their lane and kill you. So I prefer to use the mirror until the vehicle gets in my peripheral vision (which is quite good at this point in my life) so I can still watch out for morons ahead of me who are reading their email while driving. I've an Ortlieb bar mount currently but just ordered a Sehen mirror to try one on my sunglasses.