I saw an episode of "this old house" or one of the clone shows a while back, where they installed this kind of system.
it doesn't actually use ground temp water to cool your house (At least any versions I've heard of). it uses a standard heat-pump but where a standard unit would have the outside heat-exchanger air-cooled with a fan, it instead had a tank in the basement where the heat-exchanger that was normally outside is submerged in some sort of non-toxic coolant. the coolant from the tank was then piped out into the yard in a ditch a about 6' deep. every so often they had a vertical hole (looked to be about 8-10 dia, not sure how deep they went) the pipe dropped into the hole and back up, then through the ditch to the next hole, on and on until it comes back into the tank.
the fluid transfers heat much more efficiently than air, and is also at or near the 55deg ground temp.
there are quite a few benefits.
more efficient exchange due to liquid cooling.
plus in a standard heat-pump/AC unit the farther the outside temp gets from room temp the less efficiently the system works (since in AC mode you are trying to get more heat out of your house, but the heat exchanger is trying to exhaust the heat into hotter air so it has a harder time getting rid of the heat. in heat mode you are trying to gather heat from outside and pump it in, but as it gets colder there is less heat to gather out there)
so with it being cooled (or heated) in coolant that is always 55deg it continues to work well even as temperatures become more extreme.
this makes heat-pump heating cooling combo's very practical in areas where the temp extremes make heat-pump type heaters useless.
if you are just looking at the AC aspect it may not be worth the expense.
one very economical option for just AC that uses the same principle is very cheap if you access to the parts and knowledge to set it up yourself. a friend of mine showed me the drawings from a system his father had set up in their house (home made. he does HVAC). he took a standard central air unit and removed the fans and electrical from the outside exchanger and mounted it into a water tank that he had routed his pool lines through.
the A/C ran more efficiently and the waste heat removed from the house heated the pool for free.
