I have one sitting in my garage. I was very nervous about it kicking on at the right temp. I tested it and it always worked, but I was always nervous. I left it in my boat for a winter with a wireless thermometer that was tied to a display in my bedroom (I wake up worrying about stupid little things). My boat was in a tarp carport deal (full sides but just canvas/tarp, no insulation) and we were down into the low 20's a bit, the engine compartment never got below about 38*. My heater core is under my bow, I put a second Golden Rod heating deal up there. All in all, I think it worked well, but I worry about things. I also didn't have a great place to mount it, I placed it on top of my engine and then had to prop the cover open a few inches, not ideal.
It worked but I was always nervous. I wound up replacing all of my drain plugs (block, exhaust manifold, water pump elbow, etc.) with 1/4 turn valves. I still use heaters to fight moisture and for extra piece of mind. But, in the winter when I lift the boat I open the valves (takes 30 seconds) and let them drain while I wipe the boat down, then close them back up. Don't forget to close them, you would be surprised how much water your water pump can push... I also blow out the heater core by pulling one hose and blowing the water through and out one of my valves.