Right again HD.....that's two in a row I believe !!

A good helmet mirror (mounted on your glasses) is the most useful over all. It may take a whole week to get used to it. The guy at the bike shop was wrong.
The only helmet mirror I like ( I may have tried them all) is the Take-a- look. Even when this mirror is mounted on your helmet with the plastic adapter, it can move around and vibrate, making it hard to see with. However....
When mounted on the temple of your glasses, since the entire arm is steel and brass, it is solid as a rock. No vibration. If you knock it off once in a while a little piece of a Velcro strap will solve that.

You need to set it so you have it to the side, out of your main line of vision and you need to turn you head just a little to see out of it. Other wise it is distracting. Once you get used to it you can turn your head and see anywhere behind you that you like. Either side in back. You also can see the vehicles coming from a long way away, and not be surprised. You even have time to get out of the way. Give it a whole week at least, even longer if needed. I was given one to test for the bike shop before they sold them. I gave it back after a couple of days. Later on I realized that the group ride leader who was in the front could see every rider, talk to them, and see all the cars coming from the rear first, while seeing the road ahead too. So I tried it again and gave it more time.
The helmet style mirror is lighter than a bar mount mirror. The bar mount mirror only works when your head is one position. If you move your head a little you can't see the cars, because the mirror is now out of adjustment. The small glasses mount mirror works no matter where your head is. Most bar mount mirrors on the ends of the handle bars are forever bumping into things.

You still can not see ALL of the area behind and even almost beside you with a bar mount, like you can with a glasses mount.

I am no longer surprised or worried about how close something is passing or if it is moving over, or if I have to move over. I know a long time before it gets to me. Just knowing is more relaxing. Your SA will improve. YES !

All the fast riders around here, and many racers use them and do not consider them dorky!!

It's almost time to start trusting your own decisions....