** EDIT: I am happy to report that the Nest Thermostat now has a number of options for the Farsight feature, including to display the current room temperature, which I wanted, and to display the weather outside (downloaded from the internet), which is very cool indeed!
I am changing my rating to five stars, because with that problem fixed, the Nest thermostat is great!
[Original review follows]
I purchased two of the 3rd generation Nest thermostats yesterday and installed them. Generally speaking I like them, but I bumped into two problems that you'll read about in other reviews.
1. To charge its batteries, the Nest needs a C (common) wire hooked up; otherwise it tries to charge itself whenever the heating or cooling system is running -- the only time current is passing through it. When I hooked one of my Nests up, it said it needed to charge its batteries, and even though my house had no need for cooling or heating, it turned my HVAC on for hours to charge its batteries. I researched, found out what the problem was, and hooked up a C wire (luckily my HVAC controller had a place to hook up a C wire and my thermostat wires each had three extra wires in them). Good luck to anyone without an extra wire to use.
2. Much more annoyingly, Nest refuses (refuses!) to allow the thermostat to display the current temperature. During the summer, I don't have central air. If I turn the heating system off, Nest's "Farsight" feature will only display, in big letters, "OFF." It will not display the temperature. If I turn the heating system on and set the thermostat to something low, like 60 degrees, so the heat won't ever turn on, the Nest displays "60" rather than the actual temperature in the room. This is stupid! Why should I have to wall-mount a thermometer next to the Nest to replicate its thermometer, just because the Nest programmers won't offer the option to show the current temperature? According to other reviews, Nest is willfully refusing to offer this because they think the set temperature is what should be shown. I think they have every right to show it by default, but the user should be able to pick what they want -- and in my opinion, not being able to see the room temperature is very, very stupid. I'm inclined to return the two thermostats to Nest unless I hear that they're going to issue a software update to fix this. At the moment I'm emailing their customer service and they have not been helpful at all so far.
If I could see the current temperature, I would give the Nest five stars (even with the need for the C wire, because it wasn't hard to hook up).
*Edit: We heard you, Richard, and have updated the Nest Thermostat software to enable new Farsight features. It will now show current temperature as well as an animated weather screen, in addition to the other functions. We always want to hear how people want to use Nest, and we appreciate the feedback.
Thanks for the feedback about your 3rd generation Nest Thermostat.
While some systems require a C wire in order to achieve proper power usage with the Nest, this isn't true for every system. Most systems are able to provide sufficient enough power through the existing HVAC wires to be able to charge the Nest's battery. We're glad to hear that you were able to have this work performed on your system, and that it led to a satisfactory resolution.
It's true that Farsight will currently only display your target temperature, the time in analog or digital, or indicate 'Off' status. We're always seeking to improve for future updates, and your feedback is important. The best place to provide feedback that gets noticed is our Product Suggestions forum, which can be found here: https://community.nest.com/community/product-suggestions
Here's more about Farsight and how it works: https://nest.com/support/article/What-is-Farsight
Feel free to give us a call anytime 24/7 for assistance: https://nest.com/support/contact-us/