Product review (4 stars): Great product...if used with cleaning cartridge
I run my humidifier for about 8 hours a night and it puts out about a quart of water during that time.  I use home tap water and replace the filter every 6 to 8 weeks (when I see the discoloration).  I had read that using the "ProTec PC-1 Humidifier Tank Cleaning Cartridge" (about $6 each in a 3-pack on Amazon) would help and, after a year, I would definitely say it has.

The cartridge (looks like a mini baby rattle) goes into the same water pan holding the filter.  I replace the filter and cartridge at the same time because I never had success trying to reuse either.  I also clean the humidifier with a wet-wipe and let both the tank and humidifier dry out for a full day to kill off any emerging mold.  Since it costs $8 for the filter and $6 for the cartridge, I feel that $14 every 6-8 weeks is a fair deal.  I would give it 5 stars if both items were sold together, or at least by the same supplier.