Oasis humidifiers are really good and we sell around 100 of them every winter and recommend them wholeheartedly as well.
Every day we see this: Guy comes in with recent D-18 with sharp fret ends, top sunken in front of the bridge, bridge may be lifting.... strings are nearly on the frets and the complaint is that the instrument is buzzing out all over the place....
Well duuuuuuuh......
You let it dry out, Einstein....! But, but my furnace has a humidifier on it!!! Well it doesn't work very well and they never do especially if they don't get serviced frequently during the winter season....And then we bag em and tag em (the guitar not it's owner....) placing them in a bag that we have tested to be in the neighborhood of 75% RH with a couple cups of water in the bottom and a large, car wash type sponge in the bottom of the bag acting as a wick to dissipate the water into the air in the bag. Three days or so and the thing usually returns to it's original shape unless it's been dry for a long time... and then we set em up, deliver to the VERY relieved client, and all is well. Then the cycle repeats the next day....
75% is pretty high IME for an instrument to live in perpetually without something giving way such as popping a brace, bridge decided to visit it's friend the nut, etc. When we bag em for several days we are also keen to keep watch for any issues and never bag em for longer than perhaps 4 days or so. And....... this is at 75%.
As such I think that if you have RH of 75% or more frequently the humidipak is an excellent solution and when you consider the costs associated with recharging the humidipak please keep in mind that repair folks are likely WAY more expensive.... you will be without your instrument for perhaps a week, and if the image of your beloved Martin or other hanging in a garbage bag bothers you those humidipak refills may seem more economical in the big picture....
Hope this helps!
PS: IBM made their name for themselves with "FUD." FUD stands for "fear, uncertainty, and doubt" and this was and remains to this very day one of the tactics that IBM uses to push product. Slogans such as "you will never lose your job for buying IBM..." also are very much part of that FUD strategy.
Thought that I'd start my Monday with a FUD about RH.... thanks for listening.....
) guitar family instruments are intollerant of less than suitable RH and will let you know, always.... in short order.