I really like the idea and it picks up around 95% of vehicles approaching from behind. It doesn't usually pick up cyclists but I've noticed it does recognize some larger steel bikes as vehicles. It's not the brightest of lights but I think it's bright enough to be seen. My main complaint with this unit is how it drains the battery on my Garmin 520, I've turned the backlight all the way down but I still struggle to keep my Garmin going for longer rides. I would definitely purchase it again and would recommend it for rural riders.
As a tail light, this is not anywhere near as bright as some of the better rechargeables that have come out in recent years. It's sufficient for night time riding, but not bright enough to be easily noticeable in bright sunlight.
The radar feature, however, is compelling. With my Garmin Edge 1000 (I think it is compatible with several other Edge units as well), I get an audible warning when something is approaching from the rear, as well as a visual indication how fast they are approaching. This makes it very easy to glance in my rear view mirror to see if I need to move over. It can be pretty frightening when a car passes a little too close and you didn't even know they were there. If I'm riding fairly fast with a lot of wind noise, this happens to me more often than I'd like (at least before I got this Garmin unit).
It's also helpful for your riding buddies since they can easily see the light flashing as a vehicle approaches.
I'm new to road bikes and this is the light that was recommended to me. It is easy to mount and dismount and charging is simple and sure. There is an indicator light to tell you how much charge is left and the USB connection is easily accessed. I got about 2 hours at full brightness then my light began to flash so I changed it to low and had another half hour until it started flashing again. It feels tough and has performed well after three weeks of daily use. I'd buy it again.