I'm glad you posted this, I signed up on SMA's website and they sent me a free one, I'd heard of them before but didn't know what they were for
i didnt buy muslin squares but went sainsburys home section and picked up some of their value tea towels (bit bigger than a flannel but not as big tea towel) for 72p each! they wash up great and do absorb etc.
the other use i have found is to put them where baby head goes in moses basket etc and it saves changing the whole thing early hrs if baby is sick in it!