There's a lot of debate over how effective these are, but since they cost the fraction of a HEPA air filter, I basically use them as a "back up". Every bit of dust that I or my family don't have to breathe is a plus.
This concept was patented in 1987 by Charles A. McKnight. The patent expired 20 years later in 2007, and sure enough, that's when you have dozens of China manufacturers cutting up generic carbon filters and selling them as ceiling fan filters.
McKnight's idea was first commercialized by brands like Barnaki and Biodefensor, who still manufacture these today in the USA.
Barnakl Ceiling Fan Air Purifier Pads - These improve upon the original patent by with engineering that is proven to remove not just dust particles but VOCs from the air.
Biodefensor Ceiling Fan Filters - These also capture dust far more efficiently than China brands, and neutralize odors.
Thee are examples of products where the market is flooded with cheap imitations made in China, and US consumer are snapping them up, putting substantial pressure on these companies that continue to make superior versions in the USA. If you need these, it's worth it to go with the best.
