[Big Dave]

Two different things are being discussed.

True geothermal energy is using volcanic heat to generate electricity.

Water source heat pumps use ground heat to heat/cool a heat pump.

The first one is very mature technology. Geothermal heat and power have been around since the nineteenth century. It is indeed free heat, once you get past the initial cost, but there is a very limited set of usable resources and almost all of them have been developed.

Water source heat pumps are a big issue here on the frozen steppes because we have a very high water table. I can sink a 100ft well and slide a heat exchange loop down it and provide a heat sink to easily run a 5 ton (60,000 BTU/hr) heat pump. That is overkill for AC (three tons does a decently insulated 2,000 sq. ft. home) but needs help for heating. In the Midwest you need 5 BTU per hr per sq ft and that is iffy. You also need auxiliary heat to make up the difference and warm up the supply air to the 104 degrees that people like. Heat pumps are notorious for their 95 degree supply air that makes people feel cold. Water source heat pumps are very efficient and suffer no seasonal loss of efficiency.

Water source heat pumps have a future but geothermal is too site-dependent for widespread use.