Thats free heat from geothermal sources wich is not soo common.
Quite true. How useful it is depends on how affordably you can heat it. For example, the way they had it set up it was averaging 0,155kW/m² across the year. If you tried to do this with US electricity at a rate of $0,13kWh, that's $0,48/d/m², or $176/yr/m². Pretty pricey.
A couple things of note that could improve it:
1) They were raising the temperature about 20°C / 36°F, which is a huge heat increase. Conduction heat loss is linear with respect to temperature differences, but radiation from the surface is the difference of (temperature^4). Iceland is also very windy and very wet, both of which sap energy. During snows they were also melting the snow, which is a huge energy sink. I bet if one was only targeting ~10°C / 18°F, in a less wet / windy climate, energy consumption would be ~1/4 to 1/3rd as much.
2) They did no insulation of the surface. Insulating the surface should dramatically lower the energy consumption. Depending on how much insulation and what type, potentially 1/2 to 1 order of magnitude.
3) You only have to pay for more energy if you don't already have to heat something. for example, if you're already heating a greenhouse, heating it through the soil would be much more beneficial than heating the air, since soil temperature determines root activity.
4) Other forms of heating in the US would be cheaper than electric in most places, including NG and solar water heating. But of course requires a more complicated setup.
5) There's passive forms of solar heating one can do too, such as suppressing weeds with black matting, to soak up solar heat. Limited heating potential, but some.
But yes, we're very fortunate to have geothermal - it certainly makes this a lot more affordable!
More importantly, it can be waste heat. It would amazing to see some of that nuclear power waste heat be used to cultivate a huge tropical garden!
I was just amazed to see heat-loving plants like tomatoes and zucchini fruiting outdoors here, in such cold air. Really goes to show how much of a plant's health relates to those roots.