What I read which caused me to not consider this product...
I received and installed my second-gen Nest thermostat about three weeks ago and was initially very happy with it. Simple to install, beautiful, easy to use, it is certainly all that. Unfortunately, it lacks one characteristic that a thermostat MUST have. It is not robust. It fails under circumstances that a $50 hardware store thermostat would have no problems with, and when it fails it DOES NOT FAIL SAFE - which is a pretty major problem when you're talking about a device that is controlling expensive heat and cooling (and, in a cold climate, keeping your pipes from freezing!)
I was pretty surprised to come home today to a sweltering-hot apartment when the outside temperature was in the mid-to-high sixties. It's a brand-new apartment, very well insulated, and initially I simply thought that all the sun streaming through the skylights and windows was responsible for the high temperature. The Nest indicated that it was 86 degrees inside and of course, with a set point of 50 degrees, it was obviously not heating. (Right?) I set the temperature to 75 and switched it to cooling mode. Cool air started streaming from the AC vents - great. Should cool down in a little while. (Right?) After thirty minutes with the AC on, I was if anything more uncomfortable, so I went over to the Nest again - imagine my surprise when I saw that the inside temperature had gone up to 89 degrees! At this point I knew something was very wrong. It took only a few seconds to determine that the Nest had my heating on full bore - even while the AC was cooling. No wonder the place was sweltering! I called Nest and after a very short debugging session (the customer service agent had obviously seen this many, many times before) he determined that the Nest had failed in such a way that the heating was stuck on, full-time.
The rep informed me that this was a known issue that Nest's engineers were trying to solve. Apparently the Nest, unlike a cheap hardware store thermostat, is extremely sensitive to voltage "spikes" on the wires that connect it to your HVAC. Such spikes are pretty much par for the course in an HVAC environment - those wires are connected to electromechanical relays, transformers, and other old-school electrical gear - and regular thermostats are designed to deal with it, which is why you don't have problems with your cheap Honeywell. (The one I replaced with the Nest had been doing its job happily for the better part of a decade.) But this electrical "noise" can make the Nest fail - and fail spectacularly, typically by leaving the heat (or cooling) on full time, or else keeping them from coming on. Any of those failure modes could get pretty expensive if you happened to be away when they happen - you could burn through a lot of fuel and/or use a lot of electricity, and in the worst case scenario your pipes could freeze and flood your house. Nest knows this, so they are no longer replacing thermostats that fail this way (since their hardware is flawed, any replacement is just going to fail again, and the next time it could be a lot worse). Of course, they won't tell you this in so many words: the company line is that your system (because it exhibits voltage spikes) is "incompatible" with the Nest. Never mind that it is a bog-standard HVAC setup, passed their compatibility test with no issues, and has worked fine for a couple of weeks). Never mind that it works fine with a garden-variety hardware-store thermostat. Naturally Nest doesn't want to admit that their fancy hardware is seriously flawed in its inability to handle the kind of electrical noise that is found on ordinary HVAC control lines, but make no mistake about it: the fault isn't in your system. It's in their thermostat, which isn't designed to handle real-world conditions.
This isn't just a matter of semantics. The problem is that, if you purchased your thermostat from Amazon, Nest wants you to return it to Amazon (not to them). They are explicitly disclaiming any responsibility under their warranty - since it's your HVAC system that's "incompatible," not their thermostat that is broken. Presumably if the thermostat fails outside of Amazon's 30-day return window you're out of luck. At least the Nest would make a nice paperweight. This is the part that really shocked me and made me feel that the company must be getting a bit desperate. Reading the online forums, I can see that this is a pretty common problem, and I suspect that Nest financially can't handle the number of returns it would have to handle if it treated this as a warranty issue. Fortunately, my Nest is only 3 weeks old, so I can return it to Amazon, but I really hate to have to do that - it seems like an abuse of Amazon's generous return policy, when it's really Nest that should be facing up to their problem. I feel very sorry for those whose devices fail, excuse me, prove to be "incompatible" outside of the return window. At least I kept my old thermostat rather than recycling it as Nest suggests I do!
Nest has a serious engineering problem on their hands and I believe they are scrambling to do something about it before it all comes crashing down. (The rep offered to put me on a mailing list to be informed when their engineers fix the issue, presumably so that, I can court disaster again by buying the next version.) They've had a lot of really great press (not undeserved, because their device *is* really nice, when it works) but that can only carry them so far - if the news gets out that their $250 thermostat could leave you overheated or flood your house, they're going to lose that momentum. So it's not surprising that they're trying to frame this as "merely" a compatibility issue. But if I had a Nest at this point, even if it seemed to be working OK, I would still be really, really nervous about it, for two reasons: (1) the device is not robust to typical levels of electrical noise; a voltage spike could happen on any system at any time, possibly breaking the Nest and subjecting to you God-knows-what financial loss, and (2) it's not clear that the company would stand behind their product if that did happen.
Advice: stay far, far away from this product.