Executive Summary: Cool toy, but it might fail on you when you need it the most.
I bought this in April 2013 - it seemed to work fine, but then it was spring/summer and I didn't really use my furnace much. Then in October when it started getting cold I noticed my house temperature kept dropping. The nest was calling for heat, but my furnace wouldn't turn on (both house and furnace are brand new). I fiddled around with the nest and got it to work for one heating cycle by dropping the target temp below current temp (cancelling call for heat), then upping the target temp above current temp. It would work for one heating cycle then revert back to not working. This quickly got annoying. Soon after even this didn't work and the furnace would not come on. I called in my furnace company, and the tech said it was the nest. As a precaution, he swapped out the control circuit board with a new one, and away he went. Well a month later, I started having the same problem. Furnace tech came by again and blamed it on the nest. I took down the nest and put my old thermostat on and I haven't had any problems since.
On a side note, I browsed through the nest support forums, and several other people were having the same issue as me. Nest support staff didn't resolve their issue - they just asked them to give them a call. So I decided to give them a call to see what they had to say. I called a total of six times - each time the message said the wait was > 30 minutes to talk to someone; four times I hung up after waiting 10 minutes or so. One time I got through after waiting 45 minutes, only to have the rep not believe that it could be the nest that was causing the issue, then get abruptly disconnected from my call. Nest support wasn't very helpful.
Bottom line: it's a cool toy, but it's a glorified switch and one that they messed up by making it too fancy.