I, too am curious. Heat pumps got a bad rep in northern climates when used for HEATING a house, because below a certain temperature, they switch over from reverse air-conditioning to straight resistance heating, which is both very expensive and usually requires more BTUs than the resistance coils can put out. So heat pump houses from about the Virginia suburbs of the Washington DC area to parts North they are a dreadful mistake.

But....as BD points out, unless you plan on using your pool when the outside air temp is in the 30's or lower, a heat pump is going to work pretty well. And, once you are into the 50's and above, you are getting into the heat pump's "sweet spot" where it is most efficient.