I have to laugh at some of the people here saying stuff like 'when I removed the stopper the crsytals fell out' (er it's called gravity) and 'the stopper melted when I put in the microwave' (you're supposed to take it out- turn the egg upside down inside a cup before you microwave it).
These have completely removed the damp smell from my conservatory and cupboard under the stairs. If you are trying to keep a larger room from becoming damp logic would dictate you need to use more then one, also they will not stop condensation, only ventilation can do that,or repel huge tides of water (Moses is your man for that) but combined with ventilation they do a great job.
A little egg shaped miracle in fact.