I'll just observe that the thread ran from 2011-13, and there's quite a bit of change since then.
I would imagine that you could get some cost savings from a smart thermostat, but I've never used one for that purpose. The main reason is that the optimal use case for such a thing is with a family with a regular routine and no roaming pets. In our house, I'm in and out of the house at completely unpredictable times all week, my kids are watched at home, and I have to cats wandering around setting off motion sensors.
My use of a smart thermostat has always been for remote precise control. I'm extremely finicky about my temperature settings, and will adjust one degree at a time throughout the day. To put it another way: the auto settings on thermostats baffles me, as I can't imagine allowing for a 3 degree temperature window.
Instead, I just like being able to change the temp from wherever I am. If I wake up in my bedroom at 3am and feel hot, I'll grab my phone and turn it down a degree. If I'm on vacation in the summer I'll set the temp to cool to 80 and before I start the drive back home I'll set it back down to 73.
When you're ready to actually purchase one, here's what I'll suggest:
-If you're an all-Apple household and have any interest in HomeKit, go with the Ecobee 4.
-If you're a mixed household and have Google stuff, go with the Nest.
The Nest is still the best-made device. Great build quality, looks fantastic.
The Ecobee looks ok but works with HomeKit. Unlike the super expensive Nest Protects, you can get additional room sensors for much cheaper. They'll give you motion/temp, which you can use to program an average temperature for your home to use instead of just the main thermostat.
I had the Nest, but sold it when I went HomeKit. The Ecobee 4 is not nearly as pretty or user friendly, which my wife wasn't wild about. But she likes that she can now see the temperature in our nursery. Just FYI: you can disable the Alexa support entirely, which turns off the bright LED they annoyingly included on the device. I recommend the Ecobee 4 because it has better future support for whatever system you have. It can support accessories like humidifiers (which I have). Otherwise the 3 is cheaper, looks a little sleeker, and you don't have to consider the Alexa stuff.