My parents have had geothermal in their home for years. The first system was literally a prototype, and reliability was a concern after nearly 20 years of operation. They updated the hardware and now it performs brilliantly, and should keep doing so for decades. The initial cost of these systems is a little high, but the energy savings are well worth it. Electricity does nothing more than circulate the fluid through the ground loop, circulate the air through the house, and operate the heat exchangers. Alcohol-based refrigerants are now the preferred working fluid, and pose a very low risk to the environment. On top of that, you don't have to worry about the CO hazard posed by burning gas or oil.

Their house is in New England, btw, so it does have to put up with some extroardinarily cold temps in the winter. That house is always warm! I can't say specifically how much my parents pay to heat and cool the house, but they tell me it's much lower than their prior house, which was smaller and had electric baseboard heating with window air conditioners. Everything works great in the summer, too.