Product review (5 stars): Great product, and a much better value on Amazon ...
Great product, and a much better value on Amazon than buying in store (usually ~$7.99 + tax, each!).  I use distilled water in my cold-mist humidifier, but tap water for my warm-mist.  These cartridges do not completely eliminate scale buildup around the heating chamber (I think that's what it's called?), but they certainly cut down on the amount of it.  No cons I can think of.