I use a pipe cleaner cut in half and then each half folded in half again in my cobs. I have trouble with moisture in the stem and with the amber colored stems you can see it form. I am sure some of this is attributed to my technique and sometimes the type of tobacco I am smoking.
I find that the pipe cleaner filters work pretty well for absorbing moisture. Also, after a few bowls worth they do become noticeably darker, which leads me to believe they are absorbing something else other than just excess moisture.
I have pipes that I do not use this system with as well though so I'm not a filter user all the time. If I remember I will try and save a few of my used "filters" to show what I mean.