My house is under 1200 sqft and at night in winter we turn the heat down real low and just use space heaters in the bedrooms to take the chill off. We also got new, high end windows in the bedrooms so the space heaters run for about 15 minutes total. Also I like to sleep cold. Payback time is something like 10 - 15 years if it was twice as efficient given the math I did. Instead of a heater I got the basement redone as that was becoming a issue.

Part of me is also sort of thinking that each year I wait the heater I will get in the future will be a little more efficient. I've been thinking about replacing it since I moved in but just recently I'm hearing people in my area talking about heat pumps so I might see if I can give it another year or two and see if that becomes more mainstream and go that way.