I have had one for some time.... I didn't find it very useful until I got new grips similar to Ergon GP3.... I could then wear the Rearviz with the mirror on top of my wrist like a watch... then you can see what's behind you ( but at a distance the image is very small )
I use this in conjunction with the Garmin Varia radar and it might seem an expensive couple of additions to my bike but I feel so much safer.