MusicNomad Humitar ONE MN311

If you don't mind checking and refilling this little thing every few days or so (depending on the climate) it is probably quite effective. But I found it often went dry, for example if I go away for a few days. and although I always used distilled water as recommended, after a few months the sponge acquired little flecks of mould, which of course I don't want getting into my guitar case environment. Of course the sponge can be changed and you could even use a basic dishwashing sponge but then it holds even less moisture and needs topping up more often.
I bought this device because I live in Germany in an area that is very dry in the summer and damp in the winter, with temperature differences from minus 20 to plus 40 degrees. I have several guitars including a Lowden F23, and we also have a grand piano (Flügel) in the living room. The soundboard on the piano is suffering from the changes in humidity (well, it is also almost 150 years old!) and I wanted to avoid that with the Lowden. However, as I said, it requires a lot of supervision! So, if your instruments are valuable and you have the time and mindset to keep this device watered, it is worth it.
The display seems to work fine, and the device fits between the strings OK - although it feels a bit clumsy it doesn't seem to cause any problems with tuning or scratching the wood.