I find nothing beats a helmet mirror. I can scan a very wide angle behind me, and I can see more at a glance than I can with anything mounted on the bike unless it's a convex mirror. I don't like convex mirrors because I ride on roads where the traffic is moving at 60 MPH and cars that will be passing me in 3 seconds are the size of a grain of rice on a convex mirror.
I used to use a Take-A-Look, but in the course of a wipe-out a few years ago, the edge of the mirror, which has no protection on it, cut a gouge in my face that took 8 stitches to close. Also looking at the wreckage I can see that it wouldn't take much for that stiff wire to get driven into my face.
I now use a Cycleaware reflex, which seems to me to be far safer and effectively the same thing. Actually a little more adjustable, but the downside is that after 3 or 4 years the ball joint can get a little loose. Easy to fix though.
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Work: the 8 hours that separates bike rides.