we have one in our house which was already fitted when we moved in. I had no idea what it was when we moved in so I switched it off.
cue massive condensation everywhere and a horrible damp feeling in the house. obviously the fact the house is rendered and double glazed means that it has issues breathing.
with the unit switched on the condensation disappears. I used to notice the cold air being pumped into the landing. I experimented and ended up turning the fan down to the lowest setting (barely a trickle) and this seems to work fine.
in short I would recommend anyone with condensation issues to fit one of these units.