I used to have one of those mirrors that attach to your helmet visor. Handy little device, if it was a pain to keep attached to the visor. After I got used to it, I relied on it heavily to know what was behind me and to see if it was safe to move out into the lane to make a turn. I might look like I wasn't aware of what was rolling along behind me, but I knew exactly what, if any traffic was behind me. In fact, this is Robert Hurst's criticism of bike mirrors in his book The Cyclist's Manifesto (or maybe it's his other one The Art of Cycling formerly known as The Art of Urban Cycling). To the vehicle driver, it looks like the cyclist is oblivious to the traffic around him/her and off in their own world.
Well, I got a new helmet last year to replace my $20 Costco helmet. The new one was one of those fancy racing style helmets, without a visor. (Of course, after buying it, I immediately wanted a skate/snowboard style helmet). I looked at the other types of mirrors that attach to the helmet shell, but I didn't feel like gluing or velcro-ing one to the helmet. Partly inspired by Hurst, I ventured out on my commute without a mirror. After awhile I became more adept at looking behind me without swerving and I noticed something: vehicles would slow down for me and let me in a lane if I needed to when I checked back.
There are those who strongly encourage using a mirror. I'm not advocating against bike mirrors, but I find I get by safely with listening for traffic and looking back.