Product review (4 stars): Humidifiers are gross - you need this!
We have a large whole-house humidifier, and the filters and standing water in it tend to grow a pinkish bacteria after just a few days.  If not kept in check, this will permanently discolor the filter, and I can't imagine that having a bacterial colony in my humidifier is healthy.

I've found with this solution that I don't need as much as the label directs.  It's supposed to be approximately a capful per gallon of water, and I can get away with roughly half that.  As long as I've added some in every change of the water, the bacteria seems to be kept at bay.

If you have one of these large humidifiers with the cylindrical filters, I also suggest flipping the filter twice daily (typically I do this when I add water) - the bacteria seem to like the slightly drier top part of the filter, and this keeps both ends a bit damper and gives more exposure to the antibacterial agent in the water.

There are a couple of side effects, however.  One is that the filter is somewhat slippery or slimy, and has a white film on it.  It's not really problematic, except that when I flip the filter, I get gunk on my hands.  It has a sort of "bleachy" smell, which

Another thing to note is that adding this chemical causes the water to foam when I fill the tank.  I'd suggest filling the tank first, and then adding the chemical.  Otherwise, you're likely to end up with the foam flowing out of the tank and getting everywhere.  If it gets on the faucet, then you're washing whatever comes next with antibacterial agent, which might not be ideal.