I have had 3 NEST Gen 2 thermostats for more than 2 years. I love the concept. I love the UI. Their quality is poor; their reliability is poor; and their care for their customer is atrocious. Here is the litany: My first went in flawlessly and worked great for a year (more on that one later!!). My second failed out of the box. I had bought 2 at a time, fortunately, so I was able to troubleshoot by substitution pretty well, but it took close to three hours on the phone (2 or 3 calls) before I was able to rectify the situation and finally get NEST to send me a replacement unit. It turns out that the baseplate was defective even prior to shipment. Bad quality control. Fast forward one year to the first NEST again. My AC failed, requiring an expensive service call. After a couple of hours of trouble shooting the problem it turns out my heat pump was fried. Fortunately, that was replaced by the manufacturer under warranty (less than 1 year old). Further troubleshooting showed that the NEST was sending signals to the heat pump causing the AC to short cycle. NEST admits that this will happen if there is no common wire in the set up (they don't tell you that up front!!!). However, in my case, there WAS a common wire in the set up. NEST finally did send me a replacement unit for that thermostat, after lengthy argumentative phone calls. CAUTION: NEST CAN CAUSE YOUR HEAT PUMP TO SHORT CYCLE, CAUSING PERMANENT DAMAGE TO THE UNIT!!!
Now, fast forward one year more. This same unit failed again, this time after an admitted voltage surge in the house from a lightning strike. Another expensive troubleshooting repair call later, we replaced the thermostat with a "vanilla" (non-NEST) unit and there is no problem. 2 one hour phone calls later with NEST again, they concluded that the base plate had failed, but, since I was a month or two out of warranty, they would not supply me a replacement base plate, only offering me a 20% discount off a new unit (an identical offer I had received in a promotional email they had sent me that week, coincidentally). I can get replacement parts or repairs for any other piece of electronics I own. Its clear that the company has no regard for their customers.
Conclusion: NEST is a great techie product - meaning slick for gadget lovers, like me - but has horrible reliability, quality and even worse customer service.