My Jeep will have a block heater by next summer! I have a 98 with a 4.0 that has never given any starting problems. However, with a block heater, once the temp get down to 10 - 15 degrees, they sure make starting a lot easier, and warm-up time is faster. I also believe the block heater helps battery and starter life.
I have a toyota pick-up with a gas engine and block heater. When it gets bitter cold, I usually drive it to work, simply because it is warmer and more air tight. If the temp is going to be 5 or above, I use a timer set to come on about 3 hours before I will be leaving for work, If it's going to be real cold, I just plug it in when I get home and leave it on for the night (or day) while I sleep.