I’ve seen the Varia mentioned before — do you find it genuinely useful in city traffic, or is it more of a peace-of-mind thing?
I do.
I changed the notification so it's a single beep instead of the distracting multi tone beep. It beeps when the road behind goes from clear to registering an oncoming vehicle and it beeps when it goes clear (e.g. when they pass, stop or turn off). It doesn't beep each time it picks up another vehicle if it's already tracking one. Apparently it can track up to 8.
It is
very useful knowing that there are 2 or 3 vehicles behind the one you can see right behind you that the mirror didn't pick up and, a shoulder check might miss too. This helps you time a move from secondary to primary to pass a parked car or get ready for a right turn.
It's not perfect. It can "lose" vehicles if they're completely blocked by the one behind - like your eyes, it can't see through an artic! It can also lose vehicles that have exactly matched your speed for a few seconds.
Lastly, if you mount it too low and don't have a mudguard it can pick up your rear tyre as hundreds of vehicles approaching!
Post based solely on the RTL515, in combination with an Edge Explore 2 (and previously Edge 530)