when i lived in a damp house i bought one. I also had a digital thermometer (they are pretty cheap) thing which told me humidity so i used that to gauge how well it was working.
They work best when the rooms are warm (ie heating on)as they can cool and condense the water vapour from the air better, but will still work if eating is off. I just turned the dial on mine to max for a few days until the dampness seemed to have dropped to a reasonable amount (was around 60% RH on my thermometer thing) then i just turned the dial on the dehuimidifier until it clicked off and left it like that. Didnt always have it plugged in, but if i was doing some washing that needed drying in the house, or if i noticed my thermometer humidity thing getting high i'd bung it on.
I got loads of water out in the first few days then it seemed to stabilise. Made a lot of difference in my house. I just left it on the landing unless i had washing to dry then i bunged it in the room i was drying the clothes in. Obvioulsy if you have one room thats a real damp problem might be better putting it closer to that room.
No idea if what i did was the correct way to use them but it worked for me!
HTH