Two years ago, we had our oil-burning furnace, its oil tank, and our air conditioner all removed. In their place, we now have one geothermal unit that warms our air during the winter and cools it in the summer by doing a thermal exchange deep underground via a vertical ground loop system. Connections buried 4.5 feet underground bring the loops into the house.
We were nervous about the geothermal system keeping up on the coldest days (all geothermal units have built-in electric heat as back-up), but we kept our thermostat at 70 degrees all winter and the electric back-up was never needed!
Although the up-front costs are high, with all the incentives, we are on track for the system to pay for itself via cost savings in about 10 more years. More importantly, we no longer burn fossil fuels in our house nor have to worry about fluctuating oil prices.