I like mirrors; mine is a helmet-mounted one. I can scan behind for a long distance with a small move of my head.
I got so used to it that I tried to use it one day while I was out walking.

But I don't depend on it for lane changes or merging, just to scan way behind me to get a general picture. Hey, I don't depend on my car's mirrors, either. When changing lanes, nothing substitutes for eye-contact (or something very close).

The helmet mounted kind attaches with a kind of glue-patch, but eventually I had to tape mine to the helmet to
make it secure. If it's glass, don't toss it--you'll crack the mirror (I still use mine, even though it's cracked and I need a new one. I don't really notice it, usually).

Anyway, I think it's great!