We have it only for downstairs as the house has an open plan design which means that the upstairs heats very easily. We also have a wood pellet stove for the evening during cold snaps. The heating is on for 4 hours 55 minutes at night at the moment and the house is very warm, 21 degrees on a cold day.
Our last bill was €250 including hot water, lights, etc for an 1800 sq. ft house. It definitely isn't a system that adapts quickly to a dramatic change in temperature which is why it's handy to have a back up like a stove etc.