I picked up a Varia over the weekend and just had my first ride with it.
It's certainly useful and it gives you more warning than your ears/eyes ever could, even in ideal conditions. In less than ideal conditions, like today's windy ride? Forget it - completely different ballgame.
Situations where I appreciated it most were when pinch points were approaching, and I could more subtly alter my speed to avoid potential conflict, or when the streets were in really crappy shape and I could be less conservative with picking a clean line through the cracks and potholes.
I did experience a couple instances where it would 'clear' a car when it paced me long enough, and that's a little frustrating, but it only happened when the cars were close enough to be on my personal radar, anyway. If it could be made a little more sensitive to those situations without dramatically increasing false positives, it would be damn near perfect.