Product review (4 stars): Useful solution
From the bottle's label:

Holmes
Humidifier Water Treatment
Model S1705

* Prolongs the life of wick filters and warm mist heating systems
* Helps prevent lime and scale build-up
* Provides easier cleaning

My motivation for buying the Treatment was to increase the lifespan of the wick.  I was only getting a few months life from a wick, using about two per winter.  Using the Treatment, one wick is lasting more than one winter.

And the bottle lasts more than a winter, so it's worth it.

I hate to recommend W*lmart, but I bought my last bottle there for less than $2.  That was still during the end of the humidifying season.  Guess I should have bought two.  I also bought their last half-dozen HM3600 humidifier wicks for $2 each (in July), when you couldn't find them for less than $12 anywhere on eBay.

I'll need another year to determine just how much longer the wicks are lasting, now that I'm on the Treatment.  I just checked my wick.  I think I used it for almost all last winter.  It's just a little soft all over, but no serious deterioration.  And no smell, just in case you're counting.

The instructions recommend adding a capful with every 3 gallons of water -- as a start -- and then after a while, you're supposed to determine whether your water needs more or less.  I used a little less, and it still did the trick for my wick, apparently.  My water is very soft.  The instructions say harder water will require more Treatment.  Granted, that's not a lot of guidance.

I didn't know if it's allowed to combine bactericide with the Treatment, so I didn't.  Between the Treatment and being careful not to allow the humidifier to sit with water if it was turned off, the water never got smelly.