Product review (4 stars): Nothing's perfect, but these are good enough...
I've used the Holmes filters for at least a dozen years, and they work pretty well. There are many ranges of tap water, from very soft to very hard, and the water type you have available often reduces the life of these filters, especially if there is a heavy load of minerals present. My tap water seems to be in the middle of the range. I use the available water treatment products each time I add water (a bacteria inhibitor and a water treatment solution that slows lime and scale formation) and find I can get an entire season out of a single filter. Towards the end of the season I start flipping the filter over each time I add water and this also helps keep the filter soft and absorbent without too much of the crusty cling-on material forming. If your water is really loaded with minerals you may need to use straight distilled water in the humidifier, or at least cut your tap water 50/50 with it. Honestly, there is no perfect filter medium for a humidifier using tap water...but these work pretty well.