Yep, I use the Varia and while I was initally sceptically it is really useful. Particularly on faster decents when you have wind in your ears or just a plain windy day. Coupled with electric cars and cars being quieter generally I find it really good in giving me that base understanding of what is behind me (and for not worrying about swerving around pot holes or drains). I still give a life-saver look over my shoulder if I am moving across lanes though....
Only mild annoyance is heavy traffic as it is always beeping, so I turn it off on a commute in the city as there is always a car behind me anyway, don't need aradar to tell me that!