Marco Arment analyzes the Nest privacy statement that came out following their sale to Google:
If you’re using Google’s services enough to give them a pretty good idea of where you are and what you’re doing, Nest could automatically turn your heat on so it reaches the ideal temperature at exactly the time you’re most likely to arrive home based on your location, travel speed, the route you usually take, and current traffic conditions. How clever and impressive! It’s even environmentally friendly!
A lot of what Google does or could do with your personal information is really cool and clever and helpful. On the other hand, for every bit of information they collect and put to use for you, they’re putting ten bits to use for them.