Just started using a helmet mirror this week. Tried a handlebar version and didn't like it (too far down). The helmet mirror took a little getting used to, but after just one or two rides I found I was really using it, both on streets and on MUP. Very helpful IMO.

At the same time I agree with the previous poster about turning your head as an important way of communicating with drivers. So I do that too, especially when I am getting ready to change lanes or if I want to let someone behind me know that I know s/he is there.