Nothing will ruin your guitar worse than drying out: it will warp your neck and crack your soundboard. I have used this humidifier, which rests between the strings, for my classical guitar for the last six years. It is way superior to any other system like the Damp-It, which is just a regular sponge. Before getting this humidifier I had something called the Humitar for a while. You filled it up with water and the material on the surface was always damp through osmosis. It was a pain, rusted the strings and even developed mold. I replaced it with the Planet Waves almost as soon as it was introduced. The Planet Waves model holds more water than a regular sponge, so I only have to refill it every two weeks, and it doesn’t leak (note: when I travel, I remove it from between the strings and keep it in the case to make sure it doesn’t fall between the strings into the soundhole).
You will also need a hygrometer to go along with this humidifier. As soon as the humidity hits around 45%, I take the humidifier out of the case to avoid over humidifying the guitar, which can weaken the glue that holds the neck to the body and the bridge to the soundboard. I live in Toronto (a dry climate in the winter and humid in the summer), so I already had a room hygrometer and cigar case hygrometer for the room where I store my instruments. Otherwise I would have purchased the sensor made by Planet Waves (see below).
I have long had what guitarists refer to as GAS (gear acquisition syndrome). But even if you have just one guitar and aren’t a gearhead, this humidifier will help maintain the quality of your instrument’s sound, so it’s a cheap and worthy investment. And if you’re not a guitarist, Planet Waves (a division of the string makers D’Addario) also makes large and small models for other acoustic instruments.