I didn't use a mirror for 30 years, then got a Take-a-look glasses-mount mirror a few months ago. Took a few days to learn to use effectively. Now I use it constantly. However, I always do a real look over the shoulder before I ever move laterally. But it's nice to be able to take a quick glance and see there is one car coming and a gap behind it. When that car passes, you look over your shoulder and merge.

I also use it when I'm taking the lane because there is no other same-direction traffic at the time. This allows me to ride in a more visible position. I keep looking to see if anyone is coming from behind. I wait until I know they see me (they slow down and/or move left in preparation to pass me) before I merge to the right side to accomodate their passing of me (which they appreciate). I find this to be much better than simply riding along the right side, enabling motorists to pass me as if I'm not even there without even slowing down or adjusting their lane position.

And on group rides it is great to watch for that guy who tries to take a jump on you - you can see him coming and start accelerating before he blows by you-- you can be ready to jump on his wheel!