A quick note:
Using filtered water, yeah that's not going to do any good over normal tap water. But using distilled water actually has merit to it. I'm a cigar enthusiast (not quite an aficionado) so I'm concerned with humidity for another of my hobbies as well. Some humidifiers (almost all cigar humidor elements) use a material inside that's basically florist's foam. It's a funny-feeling stiff foam that happens to release water at 70% humidity, which is right in the middle of the range where us cigar smokers like to have our stogies humidified. The problem with tap water is that the dissolved minerals that are in most people's tap water tend to clog the pores of this foam over time and make it less efficient at both absorbing and releasing water. So using distilled water avoids this problem, and also reduces the probability that you'll develop mold problems.
I'm not sure what type of material most guitar humidifers use as the element inside, but any of them that use this florist's foam type of material are best served being filled with distilled (not purified, not filtered) water.
I would like to reinforce the importance of wiping off your humidifier after filling it. Not only can water drips do terrible things to your wooden instrument, but they can also promote mold and/or mildew formation.
One other note about the accuracy of hygrometers:
There are lots of analog and digital hygrometers available for very cheap nowadays, and yes they are prone to being inaccurate. But they're generally not going to be wildly inaccurate. And like Light mentioned, you're basically interested in a general humidity for your instrument's sake. With a cigar, the difference between 68% and 72% relative humidity can be the difference between an enjoyable smoke and a nightmare of cracked wrappers and difficulty keeping it lit. So us cigar enthusiasts need a pretty accurate reading on our hygrometers.
The general consensus is that even the cheapest digital hygrometer is going to be much more accurate and reliable than most of the coil-type analog hygrometers that come with most humidors. Even if it doesn't read exactly accurately, it will still let you know when a difference in humidity occurs. Here's a way to calibrate your hygrometer so that you'll know if it's off, and how much to either recalibrate it or to at least keep in mind:
Take the lid from any 2-liter bottle of soda and fill it with table salt. Just use normal, granulated iodized salt like Morton's. Now put just enough water in the lid to saturate the salt, but not enough to make saltwater. Put the salt-filled soda lid in a ziploc bag with your hygrometer. Set it aside for a while. Give it at least a few minutes, more if possible. Now the hygrometer should read exactly 70%. If not, adjust your hygrometer as necessary, or at least make note of how much it differs from 70%.
Now that I own an
acoustic guitar worth worrying about, I'm much more interested humidifiers. I still haven't found one that's wowed me with its reviews or endorsements. The Oasis ones look the most intriguing to me, in that they hold quite a bit of water, they're made of a soft material, and they hang between the strings so they're really easy to install and remove. Just tonight I grabbed an old prescription bottle, poked some holes in it with a nail, and put a damp sponge inside it and tossed it in my case. I'll have to see if that has any affect at all.
Anyways, hope this helps. Happy humidifying, and here's another thanks for this reminder post!