What Rich said. But...

I tried the take-a-look for ~2wks daily but could not get used to it unfortunately. I found the mirror and the flickering image in my peripheral vision to be a constant distraction. I was able to set it up to get a nice glance back with a twist of the head. I didn't like having to take it off and set it up before/after each ride. (I wear perscription glasses/sun glasses)

Of course one should still look over shoulder before merging.

And this is why a mirror is so useful. I find that looking over shoulder often gives motorist a signal that you plan to turn, even if you are not. This can lead to situation where a motorist will slow to let you merge when in fact it would be better if they passed and then you merged. With a mirror you can glance back, check where the right place to merge is, then look over shoulder before the merge.

At some point I want to re-try take-a-look.

Al