With the exception of one week in February 2011 where I switched back to the oil boiler to take some data before it went away, the Fujitsu 12RLS has now been heating the house for two years. The meter reads 2,584 kWh. So, about $250/year to heat House 5, in mostly milder-than-normal weather. This is about 1/4 the cost of operating the oil heating system.
Most houses in the northeast have a boiler and forced hot water heating, and most of the rest have a forced air furnace - both are central heat systems. Without some energy retrofit work, most houses can't be converted over to a single zone minisplit and have adequate heat throughout the house. In cases where the central heating system is due for replacement, a multizone minisplit may be worth considering. We've done just that at SMC, for a client with a 30 year old boiler and a poorly designed distribution system. That system cost over $20K installed, though.
A single zone minisplit costs about $4K installed. In cases where the entire house doesn't need to be fully heated, or houses in which a point source heater can carry the load of the house in mild winter weather, a minisplit can be a great retrofit. In the Pacific Northwest a major study has been conducted using a single zone minisplit as a retrofit to the many electrically homes there (http://www.bpa.gov/energy/n/emerging_technology/DHP.cfm). On average they have shown a 40% reduction in heating energy, with some homeowners experiencing much higher savings (the ones most likely that kept the doors to the bedrooms open!) The electric resistance heat is still in place, to be used as needed. It's very possible to consider a similar approach in fossil fuel heated homes. The best candidates are houses with open plans, so the heat pump can heat a good portion of the kitchen/dining/living space, and houses where the other rooms are located where natural convection (warm air rising) can transport heat to them. It would be best for the existing heating system to be one that has more than one zone, so that the zone(s) not well heated by the heat pump can still be heated by the existing system.
Best suited might be houses where a number of the rooms are not occupied - the large house with a single occupant, who needs a bedroom, bath, and the public areas heated, not the other four bedrooms and two baths. In essence, it's going back to the days when a central hearth kept the public spaces warm and the peripheral spaces were much cooler.
These changes will likely be driven by fuel prices, so they are more appropriate where there isn't natural gas - houses where oil, propane, and electric resistance are the primary heating fuels. As of the 2009 EIA energy use surveys, there were almost 9 million households in the northeast (New England and mid-Atlantic states) using those fuels as the main heating source. That's a significant opportunity.