Daddario PW-HPRP-03 Refill Pack

It is an easy system to use. Get the start kit and every 3 months (or so), replace pads with the refill pack. You don't need to monitor the humidity in the case. Very good for your acoustic guitars and hollow body electric guitars. No cracks in the guitar (due to dry out), no bending of neck (due to to moist or dry) and you can 'store' your guitar (for 3 months plus).

Only downside to this product is the price at approx. EUR 20 per refill pack, which is fine if you have 1-2 guitars, but some EUR 80 a year per guitar can be an expensive post.

A cheaper alternative is the Planet Waves GH Humidifying System, which cost EUR 8-9 to start with and no additional expenses, as it runs on water. You however need to check/refill with water on a weekly basis. This system can't cope with a 3 month resting period for your guitar.

Recommendation: keep your acoustic/hollow body guitars in case when not in use, check humidity on a regular basis (and act accordingly) OR use a reliable system. Paying hundreds or thousands of euros for your guitar and not taking care is a sin that may be punished hard - and never ever touch a crack => send it directly to a good luthier for repair asap.