Not OP. They work, but much more marginally and situationally than most people think. They're best thought of as a delaying factor in exposure, with cloth masks adding no or a few minutes and surgical masks adding somewhat more time. In many situations, this is irrelevant, because you're stuck with the group and the air for far longer than the mask differential affords. I'd say the optimal places for wearing masks are probably supermarkets, and maybe also communter transit. Can't find the damn paper now, and all information is false anyway.