Do you mean the one use ones that you get for about £6 in Boots?
I have used them a few times, both the preglued ones and the ones that come with separate glue. The preglued ones are ok so long as you never wash your hands, any water makes them come unstuck immediately.
The separate glue ones stay on better, they can stay on for up to a week, as the glue is basically superglue. With practice you can get them on straight and manage to keep the glue away from the cuticle. They are difficult to get off, you can either pick them off (bad for your nails and sometimes sore) or soak them in nail varnish remover (the false nail remover stuff they sell is just nail varnish remover). This takes hours!
You can also get them done in a salon, either ones that last, you get the upkeep done every fortnight, or one use ones that stay on a couple of days