Ineffective warning
I have a warm mist humidifier (the Vicks UV with 2 water containers, similar to my previous Slant Fin humidifier), and this cleaner is extremely effective.  Rather than filling the heating compartment five or six times with vinegar when I clean it once a week (after removing the mineral absorption pad), I can fill the compartment once for a matter of a few minutes and all the hard water residue is gone.  My water may be harder than yours.<br />Unfortunately, the instructions on the bottle say that the product should not be discarded in the sink; it should only be discarded in the toilet.  This turns out to be virtually impossible to do.  The sink in my bathroom is too small to rinse out the humidifier, so I have to rinse it in the kitchen sink.  The warning is not specific enough to tell me what the danger is for putting it down the sink.  Does it harm the tadpoles in the sewer, or what?  My solution is to run the water in the sink for three or four minutes to minimize whatever the potential is.  So I am left to wonder if I'm doing damage to my plumbing.  I'm giving only five stars for effectiveness minus one star for uncertainty.
Rating: 4 stars