I use them if I cannot get my wine, beer or mead to the temperature I want ferment at as I ferment in my basement. I have fitted mine with a simple lamp rheostat so that I can better control amount of heat I desire.
Some other posters have been critical of this procedure but it works great for me as I can keep the wine + - 1 degree on a daily basis. However, check your temp daily as I find that when my wine gets that first big rush of CO2 activity the temperature raises a few degrees all on it's own. Most of the time I have to turn the heat off or lower it significantly.