Actually, a relative has one up there....
They work quite well in northern climates. You just need more feet of ground loop for a higher tonnage system.
Some areas also have incentives for installation. If I lived about 20 miles further out into the country, the electrical co-op there will pay for the excavation for a ground loop system to be installed. Not only will they pay for the excavation, but they will give reduced electric rates ("We also have a reduced kWh rate of 1¢ from 1001-2000 kWhs each month") and offer 5% fixed rate loans for remaining installation costs....
Four 200 foot wells = four tons of capacity. The power company here gave us a $250 rebate which was about .02 percent of the cost. We could have saved some by putting in a pond loop into the lake but since we don't really control the lake floor, the installer advised against this. I'm surprised there are any of these in MN, altho they're selling in Des Moines and Ames.