Just be aware that Garmin Varia covers a range of lights. There's the Varia Rearview Radar light, which I believe is the subject of this thread, and there's the Varia Smart range which do not have the radar element, but still pair with an Edge cycle computer and will brighten or dim depending on environmental conditions and acceleration/deceleration.

Specifically the Varia Rearview Radar can monitor up to 8 cars approaching from behind and display them on your Edge or the dedicated Garmin Varia Radar Display Unit which you mount on the handlebar.

Anyway, I've just been out in the rush hour with lots and lots of traffic. The monitoring provided by the Rearview Radar was extremely comforting and useful. Just be aware that it monitors based on approach speeds exceeding a certain amount so a car may be immediately behind you but driving at the same speed as you. In this case it won't show up on the display so you still need too be careful.

I found that it gives me a bit more confidence, more awareness of traffic behind me and also makes most drivers pass more safely as the flashing light changes in intensity and frequency as a car approaches.