We got a Google Nest. It's fun! I like the Eco mode which is activated when the house is empty. It's set to maintain a temp between 69-79. It's also a very slick, ergo-friendly mechanical design. I haven't been very impressed with the machine learning aspects of the device, though, so the thermostat schedule was adjusted manually.
The city gave us a rebate of $85. In return, they can adjust the device at times of peak demand. There are some studies out there saying a Nest typically saves ~12% on heating/cooling.
When I look back on everything we've done in the past couple years, it's a lot. About the only things left on the eco wish list are 5 new windows and a Level 2 charger.