Product review (5 stars): Buy the Aircare brand name, not cheapo other filters.
I really don't get how most people write such whiny reviews about products that have nothing to do with the product itself. Wrong size? You probably bought the wrong filter. Don't like the price? You should have looked at replacement filters before purchasing your humidifier. Not happy with the seller? Leave them bad feedback, don't tell me about that here unless you mention the exact seller so I can avoid them.

We've had our humidifier for a few years now and we live practically on the Canadian border. Ambient air temp last night was -1 F (last year at this time is was -10 F). At those temps the air is very dry and I fill our humidifier about every 6 hours. I've also tried lots of different filters, the mineral additive stuff, and cleaning them. Our water is also fairly hard. So here is my take on these filters:

Get the the name brand Aircare filter, anything else won't last. The Aircare filter does not have a bad smell, as some reviewers have mentioned, and its a perfect fit for the units its designed for. Maybe some sellers are passing off cheapo filters for Aircare ones, I bought mine directly from Amazon. Yeah, so they might be a little stiff when dry and in the packaging but it really is no big deal. Install it dry, or pre-soak it, either way works and it is an equal small pain to install. I've also purchased a variety of filters from Home Depot and homedepot.com. Again unless its the Aircare brand (which is not stocked in our local stores) they are junk and only last a few weeks at most, even with the tips I listed below.

The best tips I can give besides to go with the Aircare brand filter are: 1) Do not let the filter dry out. I've found once the minerals dry into the filter it doesn't like to absorb water anymore. Don't let your humidifier run out of water. The cheapo filters are even worse for this. 2) If your filter is starting to get mineral build up soak it in water and white vinegar. The sooner you do this the better and it really works. I should take photos to show it but I'm too lazy. Cleaning cheapo filters doesn't seem to work as well as it does on the Aircare. I've tried other vinegars too but I don't think they worked as well so use the white vinegar. 3) The mineral additives do help but I'm not sure they justify the extra price and they don't work all that great from my experience. They also make the bottom tray all slimely and I never did figure out what the stuff is or if its safe. I tried the additive with a cheapo filter and still only got a few weeks out of it. I only use hot water to fill the tank because I read the hot water tank can help remove minerals. Doesn't make a ton of sense but it why not try it? I also flip the filter over every so often or after cleaning it with white vinegar. Some of these tips are not exactly confirmed but it doesn't hurt to try. Maybe someone else can confirm some of these ideas.