Product review (5 stars): Demineralizes Hard Water In Your Humidifier
We've owned our Air-o-swiss for more than a year and haven't had to replace the filter in it until recently.  It really depends on how hard your water is (mineral buildup).  For the little amount of accumulation of minerals that we have, it's worth it to just buy the extra cartridge and replace it rather than repacking the cartridge with beads.  The installation is a breeze, just make sure to soak the filter for a day before using.