They are, but I've always felt they swap one problem for another - pushing all the warm moist air into/through the fabric of the building/gaps in said fabric and importing cold air that you then have to pay to heat
if you're considering it, maybe go the whole hog and install an MVHR, and seal the building fabric up so there aren't any gaps. A more expensive exercise but one that should lower your bills in the long term and not just risk a damp buildup