After initially scoffing at the idea of the Varia radar as being yet another piece of electronics for the bike I finally grew tired of cars sneaking up on me and with a 25% discount from WesterBikeWorks finally pulled the trigger. I was an idiot for not buying it sooner. That's no joke. It's great getting alerted to approaching cars particularly on windy days where I can't readily hear them. I have the single radar paired to both my Garmin as well as my wife's so when we ride together we can both take advantage of it. The light is bright and very difficult to miss (and blinding on group rides when people don't set it to a lower intensity). The only negatives I've found is the included universal mount is so-so on my round seat post and will be replaced by the Garmin clasp type mount in the future. The other negative is you can't use it while charging, meaning on a long ride you need to be conscious of which light mode you're using so you don't run out of battery.
Bought the Varia RTL515 for my son, after first having experience with the 715 that has the rear view incident camera, on my bike. The camera on the 715 adds bulk and weight, and is only really useful to have video evidence if you happen to get hit from behind. The best thing about either light is the alert to your computer that there are cars or other bikes gaining on you from the rear. Saves your neck from having to check back as frequently, especially on aerobars. You get the alert on your computer much sooner than you would generally see the vehicles otherwise. Only reason I gave the 515 four stars instead of 5, is because my son ( who lives in another state, Oregon) reports that on his older Garmin computer, the alert is just an orange bar plus audio, whereas on mine I also see dots for each vehicle, as well as their relative distance from each other and me. I don't know if his just isn't synced correctly, if the radar on the 515 doesn't relay this info, or if it's just because his compatible Garmin computer is older, and just unable to show the approaching dots. But I know he likes it, as I do. If it's an issue with his computer, he can upgrade it, because the added safety feature is worth it.
At first, I was put off by the weight of this light vs my Lights in motion tail light. But the radar tracking of aproaching vehicles is fantastic. I've always used a small rear view mirror on my helmet, which I'm not abandoning, but that involves me tipping my head to look up every so often. The Varia alerts me to cars and even other bikes consistently, so I know well in advance if something is comming. If there are several cars in a row, the Varia shows me that as well, without my taking my eyes off the road ahead. I have it paired with a Garmin 840 edge computer.
I've seen some useres complain about the difficulty picking out specific instances to view on the cameral video aftereward. I agree. You have to scroll through a lot to find what you might be looking for, and can only do so when viewing on your cell phone. But the main reason for the camera is to document a crash or other dangerous incident. In that case , you can mark the spot on your computer widget, or if you are hit, at least the camera will have automatically recorded it for retrieval later.
It can be set to record only when the radar is detecting something, and then continues for about 30 seconds after. This saves battery, and disk memory, which is several hours anyway on flashing mode. You can get even more battery life by setting it to only flash when there is a vehicle aproaching from the rear.
For me, the benefits of this light are worth the weight, sort of like electronic shifting and disc brakes are worth the added weight over old cable shifting and rim brakes. And unlike disc brakes, you don't need to upgrade your entire bike frame to add the light.
The vast majority of the 150 Miles per Week that I ride are on roads. The Varia Radar in conjunction with my Edge 1030 Plus make those rides far safer. The unit starts to flash as soon as it "sees" an approaching vehicle. I was told by a UPS driver that he could see the flash as far back as 3/4 of a mile in bright sunlight. That anyone who claimed they didn't see it was either blind or lying. Simultaneous with the intensified flash a display on the computer screen tracks the approaching vehicle. I have found it is quite easy to estimate the relative speed of the vehicle. As many as 6 individual cars have shown simultaneously in the display. The display is tinted red until the last vehicle has passed, then it turns green indicating the road is clear. No car has ever passed me and not shown on the display well in advance. It is absolutely essential when I have to move across the lane to reach a left turn lane. This unit is my second one. The earlier one was still going strong after more than three years of use, and many thousands of miles of travel. I mistakenly thought that the battery was weakening, and chose to replace it. That is fine. I gave it to a friend who will continue to use it, likely for many years to come. Of all the safety gear I have purchased over the years, this is the best. For those who might wonder, it is also usable with Wahoo computers.