Another vote for the Take-A-Look.
When I first put it on, my optical cortex went crazy trying to figure out how to focus to see in the thing, and I also had a hard time finding the right mirror-position plus up-and-left head movement so that I wasn't always just staring at my shoulder. Now with practice I can even see behind me to the right even though the mirror is mounted on the left side of my helmet. I still always turn my head for a final check before I cross lanes or anything, but it is very helpful to be able to more easily check the rearward situation frequently in advance of making a move.
I lost my first one because it came off my helmet during a roadside mech stop. When I went back to look for it it was gone. And it was occasionally falling off. I've read some people use superglue to keep it on, but my solution is to secure the attachment with a sturdy twistie -- a plastic-sheathed one with thick wire inside, not the more common kind with thinner wire and paper coating.