Product review (1 stars): It stinks...but my nose is super-sensitive.
It stinks...literally.  I noticed an odd smell when I unpacked it, and it was only getting worse after I started using it, as the odor was now blowing out from the humidifier even though the filter was thoroughly wet.  Trying to reduce the odor, I soaked the filter in vinegar water for over a day, rinsed it out, and tried it again.  Still had the same, plastic-like odor.  I may try some baking soda, but that usually does not help when it's plastic outgassing. Please note that I am extremely sensitive to toxic odors--most everything I use needs to be fragrance-free to avoid headaches and other reactions.  If such things don't affect you, this filter may be OK for you.

11/20/15 UPDATE:  I just received another filter from this seller (Filters Now) that I've bought from them many times in the past (Filters-NOW UFK01 Kaz WF1 Humidifier Filter).  I've always liked those filters, but now they have the same HORRIBLE odor that these do.  I suspect their "aftermarket" manufacturer is using some kind of new, petroleum-based plastic in their filter material, and it smells TERRIBLE.