I use and love EVT's (Efficient Velo Tools) Safe-Zone helmet-mounted mirror. The mirror is large (2 1/4 in. diameter) and circular, the back and edges encased in plastic, on a sturdy ball-jointed stem. It's easily adjustable and holds its position well between adjustments. I've also used the Take-a-Look and liked it okay but worried about getting cut with it if I fell; after the Safe-Zone I would have a hard time going back to the Take-a-Look.
The web address is
http://www.efficientvelo.com/.