I get a little annoyed with humidifiers that state they are "care free". I don't think there is such a thing. Even if you don't have to replace or clean a filter, anything that has water standing in it I believe will get water buildup deposits that should be cleaned.
Which is what I found with this Hunter "Care Free" Humidifier Plus. Yes, I don't have to replace filters which is nice, but the slimy deposits left by the water in the tank and tray still have to be clean out. I usually end up having to do this weekly, and the thing is so big that I usually end up doing it in the bathtub so...