I recently installed a air to air HP at my second home in the Catskill region of NY. I was originally using a 100 btu propane furnace with a 4 ton cooling A configuration coil. In use about 9 years. During that time propane went from 70 cent a gal. to over $6.00. Two years ago I replaced the cooling condenser with a 4 ton 9.5 h.s.p.f. air to air HP condenser and used Honeywell's hybrid heat controller. All I can say is dam. I should have done this a long time ago.:D
I run the heat pump down to 15 deg F of out door temp and than the furnace engages heat below that. The automated controls are great. I believe I can run the HP down to even 5 deg. but decided not to because of the long run cycles.
With 410 refrigerants air to air hp's are much more efficient than there predecessor R 22 units and can run down to 14 deg f with a COP of 2.5 or so. Not bad for air to air. Large savings cost on installation over a ground loop as well.

Great info. My new place has a HP with oil backup. The oil seems the oil kicks in around ~40 deg outside, I'd love to figure out how to get it to work longer. Not as bad as propane, but oil is $$. Ouch. The woodstove is working overtime this winter.

Just ran across this quite randomly...if I understand the concept of "credits" then a geothermal system would be essentially free? Can anyone explain? https://www.energystar.gov/about/federal_tax_credits

I kind of doubt the free bit.
Up here a system is $25-30K and you get about a $8K credit. I believe you have to use a contractor to get the credit.

Often there's a catch when someone offers you a free lunch.