Nest wears the hat triumvirate of temperature commander, energy conservationist, and significant other who actually listens and responds when you attempt to communicate with it. A thermostat that makes a diligent effort to get to know you, and then caters to your every indoor comfort whim, without nagging or bargaining for "special favors" in return. In other words, the Nest Learning Thermostat is your soul mate.
After about a week of use, Nest learns and adapts to your ambient air preferences, programming itself to mimic your adjustments, and automatically increase or decrease its temperature setting based on what you've done in the days prior. If you turn down the thermostat when you leave, or while you sleep at night, Nest knows, and soon does this on its own, ensuring no energy is wasted heating an empty house on days so hectic temperature control slips your mind, and that you don't wake up doused in sweat on nights you stumble home up to your eyeballs in Cab Franc, and face plant on the bed still wearing your down jacket and wool socks.
Nest spends the first 7 days of co-habitation absorbing your habits, so it's important to be consistent during that time. By day 8, it has developed a heating schedule, which it will implement without further prompt. If you override the schedule once, Nest continues on its previously learned path, but adjustments made a couple of consecutive days, or, say, a couple of Mondays in a row, will feed into its algorithm, and create a permanent schedule change. For example, if you start working from home on Fridays, and turn the temperature up at 9:30 a.m. on that day, after a couple of goes, Nest will get it, and modify its settings to make the change for you.
Other capabilities include Energy History archives, which give daily reports on how much energy has been consumed, and a little green flit called the Nest Leaf, which appears to let you know you done good, yo, and are currently saving energy like a G6.
Nest is also Wi-Fi compatible--you can adjust its temperature manually from a computer or smartphone. So if it starts pouring rain while you're waiting for the bus, or someone dumps the Gatorade cooler on your head in January, just log on to your nest.com account, jack up the heat, and make toasty the living room that awaits you.
If you're comfortable installing a light fixture (note to rednecks: that is different than screwing in a light bulb) you can install Nest. Otherwise, professional installation is available for an additional fee.
Product Update: Nest has released an updated 2nd generation version of the learning thermostat.