Product review (5 stars): They really do work!
I clean my humidifier frequently and it always had that disgusting pinkish red mildew.  I bought these but was very skeptical they would actually work.  I cleaned my humidifier in the same way I always do and then dropped a cartridge into the reservoir after filling it back up.  The time comes to clean it again and I was sure I'd see that nasty pink stuff when I lifted the reservoir off.  I was very pleasantly surprised when there was none!  I cleaned the humidifier and dropped in the same cartridge I had used before.  Cleaning day comes around again and guess what?  No nasty pink mildew!  I will definitely be buying these again and again!