Remember that the 2 1/2 to 1 energy efficiency for Heat Pumps is at 70 degree's. As you get closer to the 35-40 degree mark, this will get closer to 1-1 like resistance heaters.
Also as previously stated, the warm air will tend to stratify near the ceiling and is difficult to bring down to the level where it is needed.
I have used them all through the years and ending up liking the radiant heaters along with the propane furnace. Radiant heaters heat objects and can be directed where you need it the most.
