We only have one for our bed; it's a king size waterproof-backed John Lewis one and I only bought one 'cos it was expensive-ish. It takes AGES to air dry though, because the moisture stays in the corners of the plasticky bits. I usually end up just remaking the bed without it so we go a week without having one at all (is only for emergencies anyway, DP and I don't actually wet the bed )
We now have a tumble dryer, anyone know if throwing it in there will actually destroy/melt it? Would be nice to just wash it and get it back on the bed same day.