I buy VERY cheap ones in bulk (I can get 2 very similar ones for £1 so I get loads!!) and replace them with a new one every two weeks.
I actualy get lots of use out of them, because, as I unwrap a new one, I then use the old one for cleaning the bath and the sink etc - they are brilliant for bringing the ceramic up all shiney!!!
I love using the poof. They seem to make the best use of soap. When I do use a bath cloth, I feel like the cloth keeps in all the soap because I don't get nearly as soapy.
I keep 2 poofs in my shower. One for my face and one for everything else. I don't like washing my face with the same thing I wash my butt with.