It works great- I'm using a heat mat temperature controller set at 41 degrees and it's kept the coop warm enough while using the 100W setting, with outside temperatures reaching the low teens. I'm sure there will be no problem maintaining 39-41 degrees if there is a sub-zero cold spell in February.
The operating cost is probably cheaper than the 60W ceramic coated bulb I was using before, since it doesn't run constantly, and I replaced the 60W with a 7.5W night light bulb. The hens sleep better without the constant bright light and they aren't agitated while waiting for me to drop the ramp. Almost every morning when I check the video feed to see when they're ready to go out, I notice they are purring contentedly.