Ours is on 17 at the moment and it's toasty - but then our house is well insulated and a semi. We only heat the living room, bathroom, spare room (because the DDs play in it) and only for those periods when we are 1) in the house and 2) not in bed. On weekdays this pans out at about 8 hours a day, on weekends it's more.
Our bill last year was about 20% higher than it was when we had milder winters, but overall it was OK. The thermostat never went up higher than 19 even when it was at its worst.
Am going to be working from home quite a bit this winter though, so am going to invest in a slanket so that I can be 1) naffly unfashionable, 2) warm and 3) a cheapskate.