Product review (4 stars): They're inexpensive and easy to install - you just unhook the cover and ...
I go through about two of these per winter season on my Skuttle humidifier. They're inexpensive and easy to install - you just unhook the cover and swap out the pad, then attach the small diffuser filter where the water spout sits.

The only downside is they're not quite as durable as some other pads on the market. They say 1 pad is supposed to last for a season, but I usually have to swap it midway through when mold and debris start building up.