I've found that my 23 hybrid gets better mileage on cold days if I use the block heater. When the block heater is used the gas engine doesn't need to run as much to produce heat. I also find that I get warm air out of the heater much quicker. Doesn't take very long before the temperature gauge...
It made life so much easier when I was using a parking garage. Best place in the world to keep a parking pass. They really should advertise this feature!