They are a bit Marmite, aren't they. As a lifelong motorcyclist I have got used to having rear vision at-a-glance through using mirrors though the effectiveness has been variable depending on what model they are attached to. Quite often I have had to come up with a non standard option.
As far as bicycles are concerned, I have found various generations of the Mirrycle to be effective for me since the 1980s. In recent years I have developed neck problems which has limited my neck movement and so suggesting a "life saver" is all very well -at junctions I can come to a halt and twist my upper body round but it's not so easy while on the move. A combination of bike mirror, hearing and using what movement I still have seems to work for me.
A few years ago I tried a Blackburn mirror (I think) attached to a helmet which in itself worked well. It made me feel a little nerdy but in fact an onlooker can barely see it unless they know it's there. It needed accurate setting up, and would clip to my helmet but needed additional duct tape to stop it from vibrating out of place -a small movement makes a lot of difference to the aim. In the end, it was the fact that I didn't generally wear a helmet that caused me to stop using it, not that it wasn't effective. Also the Mirrycle on the handlebar was always there and ready to go.
Fast forward to last June when I bought a recumbent. It is hard enough to see behind on one of these without neck issues, and I became paranoid about rear vision. In addition to the bike mirror I looked for my Blackburn one but it had disappeared somewhere in the depths of the garage. I bought a "Take-a-Look" mirror which was designed to clip on to glasses. It took a little while to get used to these but in combination with the mirror attached to the bike it works well, and I have got used to just clipping it on when I go out on the 'bent. It needs glasses with a fairly flat side arm to stay in place -if you have specs with round wire it will tend to rotate. Cycling type sunglasses would be fine. It will also clip to a helmet.
It is susceptible to slight bumps or knocks, but is easy to re align. Like other types of bike mirrors, helmet/spectacle mirrors come in many varieties.
Despite concerns from other posters about attaching things to helmets you can buy them purpose made with sticky pads or clips, made in wire or plastic. Like other types of bike mirrors some are better than others. Some designed for the US market might be left side fitting only. The one which I am using can be used on either side, is made of wire and has a plastic lens. If you wear a helmet on all your rides, probably one with an adhesive pad for fixing to your helmet would be better. I read all the reviews that I could before buying mine, and it does the job that I want it to.
As I said at the beginning, this type of mirror is a bit Marmite. It works for me as it is particularly suited for my needs while riding a recumbent. It might suit someone who, for example likes their bike to be stripped down, without anything superfluous attached to it. It's not a substitute for looking around and being aware of your surroundings, but is an aid to doing it.
