Since my wreck last year, I caution against the Take-A-Look. It's a very functional mirror, but keep in mind that it has sharp-ish edges - the pavement shoved it into my face and it left a slash that required 14 stitches. Also it has very stiff wire.

I've switched to the Cycleaware Reflex - which is essentially an equivalent mirror but addresses these problems - the mirror is on a breakaway ball mount, is surrounded by a rounded plastic shield, and is mounted on a rubber-covered, flexible wire. It's what I use and recommend now. It is ONLY for helmet mount.

I prefer helmet mount - I have very lightweight glasses and glasses mounts vibrate very badly, and also since I take the mirror off my glasses when I get there, I often would forget to put it back on. With the mirror on my helmet, I never forget it.

FWIW, I've tried a few handlebar mounted mirrors and found them all essentially useless; either way too little field of view or too convex so even large vehicles looked like pinheads. Also it's too distracting to have to move my whole head to look DOWN at the mirror to see behind me; it takes far longer than just flicking my eyes to the left for a fraction of a second, and if I want to see a wide swath, sweeping my head to the side becomes second nature; I don't even realize I'm doing it.

A few months ago I was in a store and someone dropped something glass behind me; my instinctual reaction was to glance to my (nonexistant) mirror up and to the left; I was momentarily confused before I remembered that I'd actually have to turn around if I wanted to see what happened.