I've quickly become a roadie purist, so I don't recommend riding with a mirror

Learn to look over your shoulder or under your arm to check for traffic/cyclists behind you. If you are riding with someone, learn to put your arm on his/her shoulder as you look back since this will keep you steady and help you hold your line.

It's kinda crazy, but there are so many little faux-pas that roadies use to identify other "real" roadies. As in, real roadies don't wear camelbacks, don't have reflectors, don't have mirrors, don't wear cotton, don't have tri bars, etc, etc. Show up on a group ride with any of these things and don't expect to feel very welcome. I know, it doesn't really seem fair That being said, the "discrimination" is partly because group riding requires so much trust of the riders around you, that often times, these faux-pas are red flags that a rider is new to the road . . .