My friend that I ride with appreciates the audible alert when we're riding together. He'll ask sometimes if it's alerting because he's behind me. My reply is "You wish! That would mean you're riding faster than me..." Heh.
As for also using a mirror, yes I still ride with and use my mirror regularly but I do rely on the Varia primarily unless I'm turning left in which case I check the Varia and the mirror and give a quick glance over the shoulder.