Every local rider that has the Varia really likes it. I assumed it was a gimmick when I first heard about it, but it really works.
I've been riding with one of them in small groups lately, and the Varia rarely false alarms from one of the other riders, whether they are far back or within a bike length or closing a gap. Pretty impressive. He's often the first one to call out "car back" from his Varia beep. These are low traffic country roads, I haven't seen it in action on city streets.
I've used an eyeglass mirror for years, on every ride now. It gives me an excellent view behind me. A slight turn of my head sweeps the view wide across the rear scene. But it's not often aimed right back down the road unless I'm actively looking back. So I miss overtaking cars fairly often. Wind noise makes it unlikely that I'll hear most cars, except for the loud ones, or on really rough pavement where the tires are loud.