Picked up my first Nest Protect today for the hallway outside the bedrooms, I should have the kitchen unit in a week or so. I posted in the Nest Thermostat thread.
I think because they cater to the Apple crowd (you know, more money than common sense) who will pay $250 for a programmable thermostat to save $10/year compared to a "dumb" programmable thermostat that you can pick up at Home Depot/Lowes for $35.99.
Nest has neat features, but I'd be hard-pressed to cost justify it strictly by the numbers. But there are enough Leo Leportes in the world that want the latest, neatest thing.