I purchased two of the Aqua Stones for my Recording Studio. As a musician, I own quite a few guitars. Some very expensive. Guitars being wood, can naturally dry out and crack (not a good thing). They sell many small humidifiers made out of clay or a material that will "wick" the moisture into the air and regulate the humidity.
But these are for guitars kept in their cases. I think guitars are a work of art and are meant to be displayed and played. I have used humidifiers, but in a recording environment, you can't have them running. Hence I found the Aqua Stone.
I have had mine over two years and only use Distilled Water in them. I believe this has made a big difference. I even use it in humidifiers as well. Then you won't get that salt or calcium build. But now even mine has developed a quarter sized spot of Black Mold. It's very faint but did cause me to stop using them for the past 4-5 months.
One person said that they "bleached" it and it went away. I might try that, or for the price, just throw them out and buy two more.
If I can get two years out of them with distilled water, then they are worth the price. I use a digital temp/humid meter in the room, which also has solid red oak flooring and they need the moisture up here in upstate NY and the cold winters.
My stones evaporate completely in less than 24 hours. So I fill them every day. I have recommended the aqua stones to other musicians and also to just about everyone we know because hardwood floors are very prevalent here.
Unless I install a whole home humidifier onto my furnace (which I might) this is definitely the next best thing that I have found. They are esthetically pleasing as well.
Bottom Line: Yes, I would recommend this to a friend