[billybek]

Some good points already made here, particularly about ground source heat pumps. They produce pretty much the same amount of heat regardless of the outdoor temperature so they are an attractive (but more expensive) method of extracting heat from the great outdoors as outdoor ambient temperature drops.
Heat pumps with an outdoor air over unit are much more effective now those made 10 to 15 years ago. They can produce heat down in lower outdoor temperatures more effectively than in the past reducing the requirement for back up heat.
That doesn't (in my opinion) mean that their efficiency doesn't suffer as outdoor temperatures drop. They eventually will reach a balance point where the heat required equals the heat lost from the home requiring the unit to run all the time. Their efficiency also drops as outdoor ambient temperatures go down.
That may not be a huge deal to have the system not cycle. Part of the new control strategy for heating and cooling is to have the system cycle less often, reducing energy required to start the compressor.
In very cold climates, like where I live, a ground source heat pump would be more effective albeit a much more expensive first cost to install.
My 96% efficient gas furnace will be my choice to provide heat for quite a while longer.