Here in Arizona the relative humidity is often very low, around 10-15% for most of the year. It's really hard on acoustic guitars especially, specifically ones that are made of solid woods (solid body guitars or ones made of laminate are much less sensitive to the low humidity). I have a Martin that suffered a crack in the side where the wood split due to drying out. (it also affects the frets. A dried out guitar will often have frets that stick out over the sides of the guitar from where the neck shrinks slightly as part of the drying process)

Luckily this was reparable in my situation. So I had to start using a humidifier to protect my guitars. I tried the in-the-case humidifiers from all of the popular brands but I finally decided that I should put a more permanent room humidifier in the closet with my acoustic instruments. I bought one at Sears for about $100 and it has saved me a lot of hassle and expense of putting separate humidifiers in every guitar case and refilling them every week. I keep the humidity at about %43 in that closet and it works great. My acoustics actually sound much better now, as a result.

--Jim