Product review (4 stars): Good product
Very helpful if you have hard water or humidifier creates a lot of build-up and needs constant maintenance/cleaning.  (Was easy to notice the difference when I wasn't using filter, and we have soft water.)  Seems to lessen the white dust as well.  A little expensive for something that gets used a little while and then thrown away, but aside from cost, I would say it removes some of the hassle to have one.