I own the Honeywell HEV320 humidifier. The water tank has a built in handle, that recesses into the tank. This fit perfectly in that space, would get stuck on each filling. A hassle of shaking and bumping to dislodge it each filling - aka every other day.
It was only in the water for the first 2 hours of use most of the time, since I often forgot to dislodge it. I'm sure there's more "life" in it considering this but I couldn't take anymore hassle. I switched to the Fette Filter Aquastick. It's a smaller and longer design, never gets stuck, and appears to work just as well.
If you have the HEC320, or a similar tank, DO NOT BUY THIS.