I don't use one as I don't feel the need for it with one exception. That exception is when for whatever reason I want to merge from the shoulder or bike lane either across or into car lanes. I have poor spinal rotational flexibility and the only way I can adequately look behind me is to stand up and do a quick glance back. Admittedly it's not the best method and a mirror would be better. If I lived in an area not so otherwise bike friendly I would probably get a helmet mirror.