IMO they're a bandaid for a more serious problem. If your enclosure is set up properly, manual misting or any other automatic misting/fogging system is not required. Babies do require high humidity. Adults do not require as high a humidity. Too much emphasis is placed on the number, not the results. I do not mist my animals - EVER!!
There are two main factors to humidity:
Evaporation, or getting the water into the air. This is a natural process and if the water is available, it will evaporate and need to be replenished. I achieve this with a large surface area water dish and for the adults/yearling a damp moss hide box.
Dissipation, once the water is airborne, the natural tendancy is for it to equalize with all the air it comes in contact with. You need to limit ventilation to keep the water inside the enclosure. Worst case scenarion is a screen topped fish tank. The warm humid air will rise and be replaced by the cooler drier room air. Kind of like your bathroom after a hot shower. If you keep the door closed, it stays humid, even cloudy. once you open the door, that airborne water spreads out evenly throughout your house and does not feel humid any more.
A mechanical fogger / mister has two problems IMO.
1) I've read they vibrate and can be irritating to herps. They don't like the constant vibrations.
2) You're depeding on a mechanical device to provide humidity and can become complacent. If it stops working, how long until you notice? If the cage is set up properly in the first place, simple water bowl maintenance will achieve your desired humidity.
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Thanks,
Dave Colling

www.rainbows-r-us-reptiles.com
0.1 Wife (WC and still very fiesty)
0.2 kids (CBB, a big part of our selective breeding program)
LOL, to many snakes to list, last count:
20.28 BRB
13.18 BCI
And those are only the breeders 
lots.lots.lots feeder mice and rats 