Product review (4 stars): mold appeared after maybe one week of use---think it was a fluke thing though
I don't know why I had this problem, and I'm not sure I can blame the filter because it seems like I've used this brand before without this happening.  Still, I had to change this filter out after about a week of using it only at night because of the mold growth.  Maybe I did something wrong with the unit.  I'm just saying I was quite disappointed that I didn't get longer use out of it when in the past it seems like I wouldn't see mold for maybe a month.  I don't plan to buy this brand again.

update 1/14/2014:  Okay, so obviously I still need filters for my humidifiers so I gave this one another shot.  I'm happy to report no problems.  I did use the water treatment the last time that I had the mold-come-too-soon issue and continue to use the water treatment now with no problems.  I guess it was a dud filter or perhaps the problem was with me by not adding enough of the water treatment during the original filter experience.  At any rate, I now feel okay about recommending this product.