I live in Norway which stretches from 58deg. N to 71deg. N, and temperatures below freezing for months are common. Heat pumps are very popular here; mostly the cheaper air-air pumps, but also sea-air/water and ground-air/water (geo thermal). The popularity are due to the low electricity prices here i guess.
I have an air to air heat pump with a heating CoP (Coefficient of Performance) of 4.2 at 0deg. C. (Putting 1kW of power in, gives 4.2kW of heat out.) C. At -7deg. C, the CoP are around 2.1-2.5 for my unit i think. (I might be a bit off here.
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Edit:
(There is no over unity in play here...
definition of CoP:
http://en.wikipedia.org/wiki/Heat_pump#Efficiency )
Running heat pump as an dehumidifier are, technically, not a problem, but it varies from model to model if that mode of operation are available. the heat pump at my previous house had a dehumidify mode; this one i have now doesn't, but it does dehumidify to some extent when in cooling mode (air conditioning).
Dehumidify mode are the same as cool mode, but with very little air flow through the unit; that way, it freezes the humidity without cooling too much of the air in the room.
I have little experience with geo thermal heat pumps, but i understand that if one lives in a place where the ground doesn't freeze solid in the winter, it's ok with a shallow loop, but if you have temperatures below freezing for months, going deep are the only option. Here it's normal to drill down to 80+ meters into the rock for geo thermal heat pumps. I do not think depth affect the running cost much; a pump doesn't use much power in a closed loop.
I've just installed an air to water heat pump here, since we've restored part of the house and put water loops in the floors. this heat pump will heat up an 300 liter tank wich will circulate in the floors. The tank also have a 2kW electric heater at the top to raise the 40C water from the heat pump up to 65C+ where the tank has integrated heat exchangers for hot tap water.
We're not completely done yet, so it's not yet operational.