During the warm 7 or so months out of the year that I'm trying to cool the house, I direct the cold air byproduct from the water heater into an air receiver. That air receiver is almost always drawing in air, even if it's at low speed (a property of the variable speed air handler). Because that cool air is distributed through the house, my variable speed heat pump HVAC can operate in low speed mode for a couple of hours longer in the day (no need to kick into higher gear to work hard to keep the house cool if the air is already somewhat cooled by the water heater). Last but not least, the water heater itself operates more efficiently by virtue of the fact that the air intake vent of the water heater is ducted to draw air in from the attic above it (air that's usually pretty warm).
So the free heat from the attic is used to make the water heater operate more efficiently, then the free cold air from the water heater is used to make the home HVAC operate more efficiently (when keeping the home cool). When winter comes I flip a couple of duct levers by the water heater and force the cold air byproduct from the water heater to blow up into the attic (away from our living quarters while I'm trying to keep the house warm).
It's embarrassing that I didn't think to do this as soon as I heard about hybrid water heaters.
My son rents a condo in Arlington, VA and their heat for their furnace is the hot water heater running through the condenser coil. Works pretty well I guess.