This isn't meant to be an exhaustive review of The Nest, but rather a quick rundown of what I like and don't like.
Daily usage reports (number of hours the unit ran)
Access to control temperature via the iPhone
The green leaf on the temperature display to shows you an energy efficient temperature
The green leaf on the usage reports that shows if you were energy efficient over the whole day
Beauty of the actual device (especially compared to a regular thermostat)
The fact that I am drastically more aware of energy usage habits
I Don't Like
The iOS app requires landscape orientation to view a lot of the data
2.4 ghz only WiFi (I bet the next model has 802.11ac and 5 Ghz support)
The iPad app doesn't take advantage of the larger screen. It feels like they just blew up the iPhone app.
Overall, I am thrilled with my purchase. Our first full month of using it had an electric bill of $112 (mid June to mid July). While our new house is bigger than our old one, it's drastically more energy efficient. My old house had an electric bill of $158 in the same period of 2012. I can't compare exact numbers because I never lived in this home without the Nest in the main living area, but a friend of mine did and it paid for itself in just a few months.
I don't have much to say in regards to the installation process, because my HVAC contractor installed it when he setup my unit. I'll likely pick up a second one when the next model comes out to replace the thermostat in our upstairs.