I was happy with my first generation Nest thermostat, but decide to upgrade to the third generation. I installed the E version two days ago and it was easy to do the conversion. The connector for the wires in the back is different but it was no hard to figure it out. I like the fact that the thermostat know when somebody is home even if you don't walk in front of the sensor.
I love this device! It saves me money, it's easy to use, it's convenient including the app, it's smart, it has a scheduler, etc. It has a history tab to review how your energy is being spent. This thermostat is a "no-brainer" there is nothing to think about. The device learns my habits and adjusts itself accordingly! Lastly, a lot of thought went into the design and tech of this product.
Nest is one of those products that set a standard for how all products should behave and interact. From initial wiring to configuration to daily use, they seem to have thought of everything. Apart from simply being "smarter" and more efficient, the entire experience is simply "elegant". There's never an awkward interaction or an "I wish they had thought of ____" because they likely did.