My opinion, I don't wanna know what's coming up behind to squish me into oblivion That's also the same reason I use an iPod...well ok I just like listening to music while I ride
I agree with not wanting to see what is about to squish me. I do, however, like the idea of being able to cross a lane without slowing down to look over my shoulder. The latter outweighs the decision in my mind.
Placement is strictly a matter of personal preference.
I rode with headphones when I first got on the bike. Music is a strong motivator. I quit. Not for safety, but because I discovered I like the sounds of my surroundings more. A zen kinda thing.
The only thing a rear view mirror is good for is taking a peak before you move into the travel lane to make sure all is clear. So they are handy for your actions but of little to no use when trying to judge traffic coming up behind you.
All cycling mirrors need constant adjustments some are better than others but not much.
I like having a rear-view mirror. Mine is mounted on the handlebar. It's big and round and it definitely breaks "The Rules." But I don't care. I feel safer having it.
The reason I prefer it on the handlebar is because I like the larger size and I find the view to be steady -- it's not jittery. I don't think a mirror on my helmet would be as effective for me because that style is so much smaller than what I've got mounted on the bar. And I'm not fond of adding weight to my eyeglasses. I sport specs just about 24/7, so I like to keep them light and simple.
Are you trying to decide on a brand/type or just wondering in general?
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Life's a journey--and I'm just here for the ride.
I have rear view mirrors on the road bike - very small & light, also my limestone bike - a more a full size set of mirrors - it is a requirement since I am the leader of the two person peloton that is Team Elder - I can keep my wife in view - I seldom look otherwise - we just keep track of each other - my neck is not the most flexible so I prefer the mirrors - if you want a set for you roadie let me know & I can recommend the set I have - best I have seen ! They are all handlebar mounted - I tried out the helmet mirror - wasn't digging it
She likes it they just are not for me. It is the most robust mirror we have ever had so I would say its good. It was also the only mirror I could find that fit the Trek ergo bars on her FX.
That's also the same reason I use an iPod...well ok I just like listening to music while I ride 
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