Read the DCRainmaker review and have had the same experience as Ray. Radar is easy to set up and gives me alerts on my 520 when a car comes up behind me reliably. Even my riding group likes it because they know what its telling me either because the light starts blinking or they hear the alert. In town, when cars are all around you, the radar's information is accurate but not much more useful than a bright blinking light (and this is a good, but expensive, bright light). Country roads and hills, when cars can can pop up on you, is where I've come to really like the Varia's information.
I often ride alone, usually on country roads where cars can come up suddenly. I purchased the light to pair with my Garmin 810 and have been very impressed. After riding with the rear view radar about 5 times I have become very confident in its ability to alert me to approaching traffic and even to track multiple cars. The audible alert and track display on the side of the Garmin screen is unobtrusive and easy to interpret. It seems to take a long time to recharge but otherwise awesome safety device.
I was looking into buying the older verson of this taillight for some time now. When I saw the newer version I had to have it. I am very happy that I purchase this item. Tail light works great just as advertised would not hesitate to recommend to anyone. If you are thinking about buying pull the trigger and do it great product.