Do they work?, well yes, it is a mirror after all!.

But, they are very small, so just how much help they are will be determined by how well you can take in what you see in the tiny image. My Caddy van had them fitted when I bought it, the small round ones you stick on the glass. It is easiest to use them when pulling away from the kerb to see if there are cyclists or pedestrians. But using them on the move is not so easy because of the small size. As well as the size, another drawback is that these (stick on ones) obviously restrict the available view in the 'main' mirror (and many modern cars already have quite small mirrors for aesthetic reasons). So if I was looking to fit them myself, I'd go for the type which attaches to the top or bottom of the mirror.

some years ago when I had lost the glass from the door mirror I fitted one of these circular stick-on blind spot mirrors to the centre of the door mirror housing. It gave a reasonable field of view and even met MOT requirements for driver's door mirror.

I don't have blind spot alerting system on the present car so have fitted these, one each side, great for seeing the kerb when parking as well as blind spots when driving. I find it a very useful aid to safer driving. Until I drive a car fitted with the electronic system I can't say which system I prefer to use.

Plenty of other responses about this but, as an aside, I found from experience of a close shave years ago that (if physically capable) one should always glance over the shoulder before changing lanes. I think that our motorcycling friends are well aware of this low-tech solution and it's good in a car, too!