If you aren't dealing with traffic it isn't nearly as helpfull. For me, I find that looking back will freak out drivers now and then (they assume my looking back means I will be darting out in front of them) so they take evasive action (usually slowing down). Much easier to keep an eye on the mirror till there is a break in traffic then do a quick head check and zip across. When I'm trying to get into a left turn lane and have to cross 3 lanes of 50 MPH traffic to get there, I need all the advantage I can get.

I use Third Eye the helmet mounted mirrors. Tried the eyeglass mounted ones but they caused the temple on my glasses to bend inward and put pressure above my ear (I usually wear nylon frames and they aren't very stiff). Now I just Goop the mirror onto the helmet and it's there when I need it.