I have two of these. I bought the first, a large one, and it works well when temperatures are low. You need to monitor the water level as not only does it stop the water freezing, it seems to evaporate the water over the space of a day.
During the 'Beast from the East' I stumbled upon another use for this. Temperatures had dropped to -6, and I suddenly realised that my 3 little bantams in their large eglu cube were going blue with cold overnight. They really looked peaky. I had a quilted cover, but this didn't seem to keep them warm enough. In desperation I took the yellow feet off my drinker heater and popped it into the nest box through the pop hole. It warmed the inside up just enough that my little bantams recovered. I bought a second one and now pop it in at the start of winter ready to turn on if temperatures drop too far below freezing, keeping the first one for the drinker.
This reviewer has 6-8 bantam Sabelpoots, b pets
I purchased 3 of these Drinker heaters and have finally wired 2 of them in and they work very well have not had extremely heavy frosts but on what we have had they keep the water free from ice.