Just one thing to add. While I love my block heater and use it regularly, the cost of the electricity will outweigh the gas savings. I've been keeping data for 8 months, and while I haven't compiled it all yet, I can assure you that even at 9 cents per killowatt-hour, which is what I pay, the payback isn't there. In many parts of the country the electric cost is higher. But having the almost instantaneous warmup is great, and the increased gas mileage does help to defray the cost of the electricity.

Btw, my heater pulls 400 watts, in case you want to figure roughly what it will cost you to run it for 3-4 hours per night.