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By gosh, I have had to use mine, even down here in sunny Southern Arizona. Though, not all night. I only plug it in when I get up in the morning.
IIRC the block heater is 750 watts like in my 06 so if my math is correct that's about 3 kWh for 4 hours of usage, I looked to see my current electricity cost it's $0.067 per kWh so 4 hours would cost about $0.28 so using it, it saves me $$$ not having to burn diesel to warm up the engine, by having an already warm engine I saw an increase in fuel mileage....Ditto what crash68 said. I consider whatever extra I pay on my power bill as an investment in the engine's longevity.
The new truck starts great in the cold down to about zero. I have not used the block heater at all. It has not be necessary.