I love them. Have only installed one and priced another that didnt get built. There are MANY variables that go into their selection, performance and appropriateness. Ironically, the more efficient the structure, the longer the payback. So they tend to make financial sense for big, inefficient homes or existing ones that have a good opportunity to change out the existing heating/cooling. In general, for small, efficient homes its better to spend the extra money on additional Building Envelope improvements.
"If you want to save the environment, build a city worth living in." - Wendell Berry
If you have a bad day in October, have a slice of banana cream pie. And this tiny ad: