A huge chunk of canadas population in Quebec, Ontario, prairies, and BC see -20 for weeks on end. -40 is more rare but happens every year. Huge chunks of the mid-western USA see those as well. At -30 my furnace is basically on continuously.
In terms of cost, it's a lot cheaper than gas + electricity (it evens-out on the coolest days, and is cheaper otherwise). It will not amortize the cost of the heat pump itself for a while, but that's a cost I'm happy to eat right now.