Been using one for years - wouldn't be without one!! I use this one:
http://www.dampsolutions.com.au/stor...o-dehumidifier
You can get cheaper ones of course but you really need one that supports continuous mode (no emptying a reservoir) and one that turns itself back on after power outage.
PS: The one I have can also heat (maintains effectiveness if the temp drops below 2C). I have never bothered to use heating though - not cold enough were I am plus it's expensive to run. It's also a reasonably high capacity unit. Smaller ones work but can struggle after a wet night of observing.