I have one in my house; Here's my .02
Best thing I've ever purchased to save money. I'm in and out of my house constantly for work; I work about 60 hours a week, more during the winter. I barely roll out of bed to make it to work each morning; I am extremely forgetful. I also have an alarm system at my place. Guess what is the first thing I realize I've forgotten 5 minutes down the road each day? The heat/cool is on. For the first 3 years I owned my place, my heating and cooling bills were 1-200 bucks. Whenever I realized I forgot to turn off the heat, I just said F-it and kept on going. That happened at least 5 days a week for 3 years. Fast forward to now with my nest...At any stoplight, I turn the heat on or off. For the people that say it won't save you money are simply lying. I'm paying roughly 50 bucks a month for the exact same heating and cooling I always used. Within 6 months, that paid off my nest. Now the rest is just savings in my pockets.
Anyone that says a nest is just a glorified digital thermostat obviously doesn't know anything about them. They are a learning thermostat. They are constantly updating. If for a week you used to get home at 5, and are now returning at 6, your air stops coming on at 5 and comes on at 6. For people with older systems in their house like me, it runs the fans more rather than running the heater or A/C to save energy. Heck, on cold days, an hour before I leave the office, I get out my smartphone and voila....3 seconds later, I can see that the heat has turned on in my house so that it's warm when I get home. Yes, the thermostat on the screen goes from black to orange to show you it actually turned on.
For me, it was a hard pill to swallow at first, but it's worth every last penny and then some. Anyone that says otherwise either has a state of the art a/c and heating system, an envelope house, or just doesn't have a clue. Before the Nest I was probably in the top 95% of people that wasted energy a year; afterwards, when I get my Nest report each month, I'm in the top 5% of energy SAVERS. That's huge. HUMONGOUS! I could go on and on about how great the thing is. Buy one.