I use my block heater every night that it goes below 40 degrees. It not only helps the engine but will help the trans out too.

I have mine hooked up to a HD timer/relay to start heating the engine at 3:30 AM. When I leave for work at 6:30 AM the engine block and coolant is at least warmer than a dead cold start. With temps hovering around 0 deg F that last week here at night, the car has started and idled much better.