Product review (3 stars): They work, but they don't eliminate all of the mineral residue.
I have very hard water here in Rockland County, NY and have just come to accept it as a part of life.  Part of that acceptance is the fact that my humidifier will always have mineral deposits in the form of white powder on its exterior.

These humidifier tank cleaners promised to rid me of the white powder mineral dust.  The packaging says they are non-toxic and emit no odor, and indeed I noticed no difference in the mist coming out of my humidifier.  In fact, I also noticed only a modest difference in the amount of mineral dust coming out of my humidifier.  There was definitely less of it, but it was not eliminated completely.

Using the tabs is easy, you just drop one into the water tank and replace it every 60 days.  I have a Roolen ultrasonic humidifier and the spout for adding water is not that large, and these tabs are, so it just fit inside the tank's opening.  It will be interesting when the time comes to remove it...

All in all... they do help, but they don't work miracles.