Product review (5 stars): If you use one of those sponge/paper wick humidifiers, buy this.
I'm pretty sensitive to contaminated air... This really removes any musty odor.

If I don't use it, after a few refills I'll start to notice a sort of "thickness" to the air coming out.  This 100% fixes it, and you only use a tiny amount.  I was using less than they suggest, often only using it during every other refill and I still had great results.  The first bottle lasted about two years, keeping my house at around 45-50% during long and cold New England winters.