Block heaters are a blessing in very cold icy environment like some parts of Scandinavian countries, Canada, Russia etc. If combined with coupe heater, they provide warm engine and interior before you set out on your journey. They provide for easy engine start, lower emissions due to warm engine (less fuel consumption), less wear and tear of the engine and comfort (warm inside, no mist or ice on the wind screen, easy start). And that applies even to petrol engines, not just diesel engines. There are two kinds (or may be three) of engine heaters. One in which the electric rod is inserted in to the engine cooling system through a drilled hole in the engine block (heater rod in contact with the coolant fluid), the other where the flat heating pad is applied to the out side of engine block (less efficient but a cheap diy solution), and finally a heater rod inserted in to the motor block but not in direct contact with the cooling fluid. There is yet another simple DIY system where the heating rod and a small pump combined is connected in line to the coolant hose going to the radiator. You just cut the pipe and fit that in line. The electrical heater and the pump in line will circulate the warm fluid making the engine warm. The question to ask here is what am I warming, engine coolant fluid or the engine block with the engine oil. Having used the first kind (with coupe warmer) on my Toyota Corolla before when we lived in northern Sweden, I still miss it. We now live in the south of Sweden and the winter being shorter and less intense do not need it.