I've found that most do more harm than good. When we ran a humidifier in our bedroom I would have to sleep with a blanket covering my nose as the moisture would gather there and it would get run and get very cold.
The first year we lived in our house we found it had been smoked in because the moisture from the humidifier sitting on the dresser by the wall caused the wall to bleed nasty brown stuff. Its no wonder we were sick so much, we had to buy a sander and sand that stuff out of there then paint everything good.
Now we've learned to only use the cool mist ones because the warm ones just breed nasty crap in them and the heating element rusts out wfter the first use and lets all the rust in the iar too since they are all made so cheaply.