Product review (5 stars): gets yellow very quickly
The filter seems to work well although I am not sure how to determine this. We have very hard water in our area, so the filter becoming yellow quickly could be the reason for this.  I have not noticed sediment in the water for a while, and there is no evidence of mold in the humidifier itself, so can only assume that this means the filter is working.  They are easy to install (just drop in), and are not very expensive.