I've got my thermostet set to 17C now. It was 18C, but I lowered it, and no one has complained.
Socks and a light sweater are a must, but it's generally quite comfortable.
I keep things at a constant temperature as we have a geothermal unit, and it does not deal well with abrupt change. Electricity last year was quite expensive, so I'm hoping to be a bit more of a badass this year (the geothermal unit is powered with electricity, so I can't isolate heating costs, but I can definitely say that my hydro bills last winter were more than twice what they were during shoulder season, and way more than during cooling season). My bills will always be high though, as I have a sump pump that runs year round, and I use a dehumidifier in my basement to keep the moisture level from going too high. Plus I'm heating a large bungalow.