They are pretty common in Europe, because older houses here all had gas-powered furnaces, and with an air-to-water heat pump you can continue to use the existing tubing and radiators.

New house builds often include one to run domestic hot water and under-floor heating for bedrooms and such. The downside here is that you probably also need an air-to-air heat pump for cooling in summer, and now you have two expensive heat pumps, and a whole extra set of pipes running around...