I've heard that they work well too. I actually made one in March when I got my baby chicks.
I live in Michigan and I just wonder if it "really" will keep the water from freezing. I do have a different set up that I like much more, but I'm going to take this cookie tin one I made and wait till the dead of winter and place it and a covered bucket of water outside and see if it "really" works. I just find it hard to believe that there is enough heat generated from a 40 watt or 60 watt bulb to even warm the cookie tin, let alone the bucket of water................ But they say that they work so we will see.
I've been using one for going on 4yrs with a 40W bulb, and I don't think I'm imagining things when I say that it works.