Reviews on Nest Thermostat - That Learns
I've got one that I've been using since late December. I'm not sure yet if it has helped reduce heating costs.  I have to examine the bill numbers from the last two months.  

I also switched operating modes on the furnace when the Nest was installed.  I was previously using a single stage thermostat and relying on the furnace (a Bryant furnace) to decide when to activate the second stage.  With the Nest (and a new wire for activating the second stage (see below)), I'm now letting the Nest control both stages. So, I've changed two variables that will make deciding if the Nest is winning a bit more difficult. 

It is easier to use than the last "dumb" thermostat we had.  There are other brands of smart thermostats out there that are similarly priced, and I'd bet they have similar features and smarts.

Installation was as easy as other thermostat installs I've done, save for the need to run a new control cable from the furnace to include a common wire. Our existing thermostat included wires for the R, G, and W connections but didn't include the C wire. This prevented the Nest from charging properly. Luckily, this new cable was very easy to run in our house, otherwise it would have been an upsetting development. 

A couple things I like about the Nest in particular (these may be present in other brands, too):


Control via Internet.  Through the Nest website and an iPhone app, both my wife and I can adjust temperature and schedule. This is nice if we are on the way home and want to make sure it's not cold, if we wake up early and can't bear the thought of getting up when it's cold...
it provides visual feedback regarding whether a new temperature will save money with a green leaf icon when you decrease the set temperature.  It's not rocket science, but it's nice to see the leaf. 
It's pretty. It's nice that it doesn't look terrible on the wall.
There is apparently a ZigBee controller in the Nest, which suggests potential for some kind of future home automation integration.  That's all speculation based solely on the existence of the chip in the Nest as found in a teardown, so I wouldn't bank on that. 


I think the ease of use and the Internet control are worth the added cost over a normal thermostat.  They are nice to have. But, I think that a properly programmed normal thermostat will be as efficient as the Nest.