Product review (5 stars): works for William humidifier
I use this with the Stadler-Form William ultra sonic Humidifier.  You are suppose to soak this in water for 24 hours before first use.  If you have soft natural water, it will last up to 60 days.  If you have very hard water it will last about 10 days.  All of this is assuming you run a full tank each night.

This is a must if you use an ultrasonic humidifier.  If you don't change the filter you will get white dust.  The best way to test if you need to change the filter is to put a mirror up to the mist and see if you get white dust on the mirror.

If you own a Stadler-Form humidifier, this filter will work for you.