Product review (5 stars): Works wonders!
Our humidifier would frequently get a "pink slime" in it, even though we cleaned it often, and even ran bleach through it every 1.5 months or so.  Not only does this water treatment help prevent slime build up, but the inside of the humidifier looks cleaner and even the filter isn't getting stiff like it was.  Previously, every 1-2 weeks I would soak the filter in vinegar-water as it would otherwise become stiff and stop soaking up water.  This water treatment even alleviates that problem, making the filters last longer between replacement.

Since you only put in about 1 capful per humidifier jug (varies on humidifier capacity, of course), it lasts quite a while.  I definitely recommend this for those with humidifiers.