Product review (5 stars): Much cheaper than replacing the humidifier!
Our humidifier stopped working after two or three months of almost daily use.  I was about to replace it when it finally occurred to me that it might be a filter issue.  Sure enough, I found the very gross filter when I detached the lid.  I got my replacements quickly and the humidifier is now once again working like a charm!  It's handy to have two filters in one box.