They are great if you are looking to use electricity to heat because they are 3 to 4 times more efficient than resistive heaters. It depends on the price of electricity and other heat sources in your area.
If you live in the north where temperatures go below -15 Celsius, air-to-air units aren't as great because they lose efficiency as it gets colder outside. My heat pump has a COP of 3, meaning it can generate as much heat as a resistive heater using 1/3 the electricity, but it shuts off at -15 Celsius because below that point it is less efficient than resistive heat. If your heat pump has a COP of 4, you might get to -25 Celsius before it is less efficient than a resistive heater.
Better, but more costly, heat pumps use in-ground water, either from a pair of wells, or from a deep pond that won't freeze to the bottom, or from many meters of water pipe buried deep enough beneath your lawn to not freeze in the winter. Because of the depth, they always have access to the earth's heat so they don't lose efficiency in the winter.
The Technology Connections YT channel has a couple of good videos about how them:
1) their owners insist they are the greatest thing ever and
2) nobody in the midwest seems to sell them (at any price)
I don't quite know how to reconcile those two things.