Terribly inefficient if you like to have the cabin heater set at 21-23 throughout the year. If you're willing to bump it down to 18, and use the heated seats/steering wheel, and maybe even wear a jumper, you'll get much better efficiency, in my experience. During the winter months, I only get around 1.5-2.0mi/kWh in the former scanario, but closer to 2.5-3.0mi/kWh in the latter. ECO mode is great too, but I find that my feet are too cold, so I only use it on shorter trips, where I don't want to waste a load of energy heating the cabin.
The heated seats and steering wheel use a negligible amount of energy compared to the cabin heater, so try to use those as much as you can.