After air conditioning and heating costs, the energy used to heat water becomes the largest energy cost in the average home. So reducing the amount of energy used to make hot water can be a big deal. GE estimates that compared to a standard electric 50 gallon tank, a GeoSpring can save a household $320 per year in energy costs (assuming a electricity cost of 10.65 per kWh, the national average).
The GeoSpring is very expensive at about $1500, compared to regular electric water heaters ($350 or so for a basic model), but you can get some federal tax credits (and in some states, state tax credits or rebates too), to offset the higher upfront cost. And after a few years of energy savings (which will increase if/when the cost of electricity increases), you’ll be in the green.