Product review (5 stars): no deposit buildup despite very hard water (will update if that changes)
So far so good -- bought this to use with an ultrasonic WARM mist humidifier (slightly less common than cool mist: it boils the water before pushing the vapor into the air), in a house that has very, very hard water. In the past scale buildup and caking inside the humidifier unit has been a huge problem. But this product used as directed (2 small capfuls into every gallon of water) *seems* to be helping keep the humidifier clean. (It's only been in nonstop use for about a week this year -- but by the same waypoint last year the humidifier already needed aggressive scrubbing.) Update 2/13/14: gotten through almost a whole winter now without having to de-scale the humidifier -- amazing! upgrading to 5 stars.