Product review (5 stars): Removed the stink from our filters!
How did I not know about this product until recently?  A few weeks after purchasing and setting up my vornado humidifier, my daughter's nursery took on a terrible odor.  We thought it was her diapers (we cloth diaper and have a wet bag permanently in use in her room), but it was more of a musty, dirty gym type smell that I finally figured out was coming from the humidifier.  After replacing the filters twice, I began to wonder what was going on.  Finally after 2 years of dealing with this intermittent smell I discovered this water additive and it has completely removed the filter stink.  I'm hoping our filters will last an entire winter and maybe even multiple years like another reviewer commented!

*** UPDATE ***

13 months after posting this review I have still not had to replace the humidifier filters!  I just ordered a new bottle of this since our old bottle is almost gone.  This amounts to less than $5 for 1-2 years of scent-free humidifying.  This product is definitely worth the price and saved us from spending a LOT more on new filters.  I highly recommend!