Large buildings already have control systems that do this, and Honeywell manufactures many of them.
The "Nest" device may well be mostly hype. (What is "far-field motion detection", anyway?) There's only so much you can do with input from one location and nothing but on/off control over heating and cooling.
Compare the EcoBee, which does the same job, and probably better. EcoBee can handle remote sensors for outdoor air temperature. It measures humidity, which "Next" doesn't claim to do. It can be set up to control fans and dampers. (One of the biggest wins in HVAC management is figuring out how much air to take from outside and how much to recirculate.)
Nest is a status symbol, not a HVAC management system. It looks cool. It creates the illusion that it's doing something "green". It probably helps a little.
Look at the EcoBee, and without reading any instructions or manual, attempt to change the temperature lower or higher. Do those "menu" type buttons do that job? Or is it a touch screen? Those are not immediately obvious, and most of the population would say the same thing.
Nest is an attempt at making the interface in such a way that the usage is obvious to most of the population without looking it up in a manual. Right now, that costs extra, but maybe not for long.