As long as the temperature doesnt drop to freezing or below, its a total waste. I'm gone all day, so for me to run one space heater in a bathroom for 30 mins, and another in my home office for 4-5 hours an evening, and all weekend have saved me a ton of money.

If the temps drop to below freezing for more than a few hours though, fire up that oil burner, cause paying for oil is WAY better than paying for all new plumbing!