This humidifier works as specified. I have no major complaint except that I slightly prefer the similar product from MusicNomad. The one from MusicNomad has a hinged lid which is much easier to open for checking the condition of the sponge when the humidifier is installed between the strings. The lid on this D'Addario humidifer has no hinges and sits so tight that it can be difficult to open without removing the humidifer completely from the guitar.
Also, the sponge in the MusicNomad humidifier seems to hold slightly more water and doesn't need a refill quite as often.
These are just minor complaints though. This D'Addario humidifier works fine. It does exactly what it's supposed to do and will keep your guitar safe from cracking during the dry winter months, provided of course that you keep it inside a case. This humidifier will be more or less useless outside a case (but so will all similarly designed products).