I haven't tried using them before, but I've heard too that for some people the lack of stimulation means a reduction in milk. For others, it makes no difference.
I've also heard of people buying them because they have damaged nipples but they find it still very painful, even through the shield.
I've also heard that if you wet them first they can stick on better.
But yeah, I haven't tried them so I haven't any first-hand knowledge