Just add to this. I use it with the Wahoo Roam. I'm going to upgrade to a Garmin 1030 for a variety of deficiencies of the Wahoo computers, but one really handy thing on the Garmin computer is that it allows control of the lighting profile. The Varia and my Bontrager lights would be so much handier if I don't have to click through modes at the start of every ride, always hard with a winter gloved hand. The only good thing I can say about the Roam and Varia combo, is that if someone were hearing challenged, the LED mode Wahoo implemented is probably very useful as it's more noticeable than the in-screen alerts. But I go totally by the sound, then look at the LEDs.

I consider the Varia a must have. The mirror plays tricks on me while the Varia never fails. It is analogous to my backup cameras on the car, I have missed things looking around which the camera sees and it's saved me from doing something stupid more than once. I won't ride without a Varia now, I have mounts on all my bikes and transfer it. Once an electronic device is proven, it's more reliable than our senses. It has warned me even of approaching cyclists. I've had no false positives or negatives in three months of use. Once a car surprised me and that was because of it being on continuously in town, I count every car it senses, and monitor when they pass me, but I must have screwed up. At that time, I was preoccupied with potential opening doors in the bike lane and would give the Varia the benefit of the doubt, assuming my bad.