I like the idea behind this. The reason why I never use things like this or mirrors is, I wouldn't want to be reliant on them. How effective are those mirrors at night?
I use the glasses mounted mirror and they work just fine during the day, at night they're just like using a car mirror at night, they work the same, I've used mine at night and never had any unusual issues, it's a mirror, it works like a mirror, it works like your car mirrors.
Being reliant on things like mirrors? Are you reliant on your car mirrors? Your car mirrors are your eyes for the sides and rear of the car, of course you should also always check with using your head and turn it so your eyes can see directly. Ok, so now you have a mirror attached to your glasses or helmet or bar end, or maybe a combination of two, but guess what? you should still check by swiveling your head as you would in a car. Ok, now instead of a mirror you have the Garmin radar device, guess what? you still need to swivel your head to see what's going on to your side and behind you!
Campybob is right when he said to use more than one light for the rear, its what I do as well. However a bright tail light will obscure any light output coming from that little Garmin 9 lumen light, so I wouldn't even count that Garmin light as a light. I have sort of a "triangle" of lights on the rear of the bike, my brightest one is mounted to either the seatpost, seat bag, or seat tube depending on the bike I'm using, I then have a light on each rear seat stay, and one on the back of the helmet. The brightest one is set for steady the rest on blink or flash mode. The reason I have one on steady and the rest flashing is so that rear coming motorists can ascertain their distance from me better, and the rest flash to attract their attention. in the daytime I use the brightest one only and it is then on flash. Of course that's how I do it which doesn't mean you have to do it that way. I've been riding so long that I remember using tail lights that were amazingly dim, and used lights like that for years and never got rear ended, but in today's world of distracted driving you need to catch their attention.