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.
To complete the discussions here are two Scandinavian suppliers who have two different approach.
(1) Defa has a plate heater that is glued on to the engine oil tank at the bottom of the engine. This is an easy DIY technique that heats the engine oil and allows for smoother running of engine when cold. But there is a lot of heat loss because it is applied externally to the engine oil reservoir or the engine block. I would apply some insulation blanket around it to reduce external heat loss in severe winter. http://old.defa.com/vfp/eh/414890.pdf
Which one is better`? The best one would be a heater rod inserted in to the engine oil reservoir but it does not exist as far as I know.
All this can be combined with a electric timer and a coupe heater (for your personal comfort) and a battery charger that tops up your battery for cold start.
Dipstick heaters for the oil used to be common and cheap but I dont think they work as well as oil pan heater pads because they are too small.
For someone that might want heat for a temporary deep cold spell magnetic oil pan heaters can work. I have a hard time finding adequate flat space on the oil pan because the plastic engine bay splash shield but it does make the startup quieter.
Its easier to park in the garage so thats what I do.
Unfortunately apartment dwellers might not have the AC power outside for these heaters.