Anecdotally, a few people I know who moved to New England (where it can get down to -20F at night, though rarely) have installed heat pumps and been happy with the performance through the past few winters, though I don't know which models they bought (one mentioned the Mitsubishi H2i).
I believe all of them also have a woodstove for backup heat and general coziness, but no separate fossil fuel backup.
I believe all of them also have a woodstove for backup heat and general coziness, but no separate fossil fuel backup.