Questions about energy-efficient heater/air conditioning system

I had to have my air conditioner serviced last week; I'd gone as far as I could poking around in there without causing potential damage, so I called for help. As we settled our business at the end of the service, we chatted about newer, far more efficient models of heaters/air conditioners. They are pricey but there are often rebates available, and the service guy (what's the right job title? I dunno) said they pay for themselves very quickly. I want to quantify "very." So here's the third in a series of questions that don't yet have answers. Feedback, anecdotes and suggestions welcome!