There is a ton of info out there on these systems buit people need to pay attention to the specifics of which system they are talking about.  There are "Air Source" and "Ground Source" geothermal heat pumps. Both function in a similar way but each has a different source for where they extract the heat from. 

Because air source heat pumps rely on the ambient outdoor air, they don't work very well once temps drop below freezing. They'r eused widely down south but don't work very well up here.

Ground source heat pumps rely on sub-surface water for the heat exchange.  Because ground water maintains a fairly constant temp once you get down far enough, they are a lot more reliable source of heat.

They're expensive as sin to install but they are very efficent to operate.  The industry claims savings of 40-70% on heating costs and 20-30% savings on cooling and, of course, since you aren't buring fossil fuels, they're considered to be very "green".