We adopted a cat that was wearing them at the time of his adoption. They worked well as far as he didn't do any scratching damage while he had them. He didn't try to chew them off either. We let them grow out and off and never put new ones on because our home is pretty much cat friendly.
They are like rubber covers that are glued on. They have to be replaced when the animal's nails grow out.
I would guess it would be MUCH easier to put them on a dog rather than a cat!