I'm also a helmet mirror user. I tried glasses-mounted mirrors, but gave up because they drastically changed the balance of the glasses I was using at the time, causing them to easily fall down my nose. With the better glasses I have now, this probably wouldn't be a problem, but I haven't gone back.

I don't think it has been mentioned yet, but many people mount helmet mirrors too far forward on their helmet. This makes the mirror's apparent size really small, showing a tiny view of what is behind you, and makes it hard to adjust so that you can see behind, rather than behind and pretty drastically to the left/right, without also seeing big chunks of your head at the same time.

For maximum effectiveness, you want the mounting point to be well back on the left or right side of the helmet, such that the mirror surface is separated from the nearest eye by no more than around three inches, maybe less.

The mirror should be adjusted so that with the eye farthest from the mirror, you can see about half the mirror past your nose (the other half would be obscured by the nose); and with the eye nearest the mirror, you can just see a bit of your ear in the edge of the mirror. The view with that near-mirror eye should be directly behind your head. Obviously, these mirrors are not meant for binocular vision.

With the mirror so close to your near eye, you will be able to see a pretty wide field behind you. As others have said, turning your head slightly lets you sweep up a pretty vast area.

Vibration of a properly mounted helmet mirror is distracting at first. It took me about two weeks to get used to. However, if you look carefully, you will see that vibration of the mirror itself doesn't cause the image to move about all that much. Much of the battle of learning to use a helmet mirror is learning to ignore the fact that the mirror frame is jerking around, and pay attention to what the mirror is showing you instead.

I use mine in all rides, including crowded benefit rides where I'm in a paceline or peloton, and find it very useful. To the specific question of what good it does me: it helps prevent me from doing something stupid. Sometimes there are approaching cars that I don't hear (perhaps because of other noises in the area, like construction); sometimes on a path there is a faster rider overtaking and s/he doesn't call out to warn me I will be passed. In either scenario, if I decided to make a turn in the wrong direction, I'd be toast. So I consider it largely stupid rider insurance, where the stupid one is oxygen-deprived me. But I also use it in small, informal duo or group rides; I can easily check to make sure that I'm not leaving my partner behind, and keeping in contact and staying friendly is that much easier. I just find it really handy in several different ways.