The literature seems to indicate so. "Regular" heat pumps just use the differences between the air and the house and so only work well in fairly moderate climates. Geothermal ones use the difference between the earth and house and at a certain depth the ground's temperature doesn't change much all year. It would seem the greater the difference between the house and whatever you're using to heat or cool the house allows for greater efficiency in the system. The geothermal systems, at least, can be used to generate both heat and cool.
Thing is, the geothermal systems use a heat pump kind of like any other, but you have to drill into the ground to put the tubes the heat pump circulates fluid through... so the heat pump is more expensive and you have to invest in the drilling. Most of the literature seems to indicate you can pay it off in about 6 years, but I'm basically curious to get some first hand experience to evaluate the literature against.