I've used this heater for the past year in Northern Wisconsin, where the weather can go to fifty below. Just put it on the oil pan and plug it in. It maintains sufficient warmth and oil viscosity to start the engine the next morning despite the cold.
I bought the smaller one and it's worked great. If I had one complaint it's that, while the unit functions in extreme weather, the electric cord insulation cracked due to constant use in sub-zero weather. I will replace that, since this unit is indispensable to someone living in a very cold area.
I have a specialized engine block heater in one of my cars and have used this unit on the others. It functioned as well as the engine block heater, despite it's small size.
I would highly recommend this unit. The magnet is plenty strong to attach firmly to a flat metal surface and the small size is actually an advantage when sliding it beneath the engine to attach it.