I finally had a chance to really use the Varia on the road for hours at a time when I went on a 6-day bike tour last week, and I have to say that I am very impressed with the way it performed. I also have a mirror, but the Varia picks up cars before they're even visible in my mirror, plus the "beep" from the Edge is enough to alert me to the fact that someone is overtaking me (although a couple times it was just someone else in our group catching up with me!). Being able to detect multiple vehicles is also helpful since I've made the mistake a couple times of not realizing there's someone hidden behind the car that I can see in my mirror.
This trip also let me finally figure out how the sleep mode works on the Varia -- or at least I think I figured it out. When I take a long break and put the Edge to sleep, the Varia also stops blinking and goes into sleep mode. Then when I power the Edge back on, it also wakes up the Varia and everything reconnects. I had been wasting time powering the Varia off separately and then getting into connection problems by losing track of what mode things were in, etc. At one point the "light network" functionality appeared to be lost, but going into the Sensor settings it had apparently just become disabled so I was able to get everything back.
Anyway, I find it amazing that they can pack that technology into such a simple-appearing device and it was well worth its price IMHO.