The thing I still don't understand about the Ecobee sensors is how they help the house as a whole. Sure, putting a sensor in a room that doesn't heat up as quickly or bleeds heat will result in the system staying on until you reach temperature there...but you're still only controlling one furnace. So while it's blasting heat to warm that cold room, there's still hot air going everywhere else and when your cold room isn't cold any more, it'll be unpleasantly warm in the other rooms (and you'll have burned a bunch of extra natural gas).
I would think it would be more efficient to pair any smart thermostat with some manual tweaks to vents to help distribute heat more evenly around the house. I mean, I've bumped up our Nest by an extra degree to help account for the two colder rooms at the back of the house, but the side effect of this is that the front rooms get too warm, and no amount of extra sensors is going to fix that.
The sensors work when you are going to be in one part of the house for a long time and you don't care about the other zones.
For example our bedroom is hotter in the summer and cooler in the winter. At 9pm every night the Ecobee switches to just using our bedroom sensor so the bedroom is at the temperature we want. That might make the family room hotter/cooler than we would normally want but since we aren't there we don't care.
To help keep air temps somewhat consistent in the house I have it set so the furnace fan run at least 20 minutes an hour.
A few companies are starting to make smart vents that can open and close. From what I have read on HVAC sites they can cause issues if they close off too many vents and don't let enough airflow through.
One huge advantage of either thermostat is you are on a real time pricing program for electricity. We are on ComEd's Real Time Pricing Program and we pay the rate of electricity based on the hourly price. Yesterday was a 'bad' day for us on the program but over the course of the past year we have saved $260 vs being on the standard pricing plan.
Where a smart thermostat can help on this plan is in the summer. We typically have our AC set at 77 degrees in the summer. On the real time program, ComEd recommends getting your home to five degrees cooler than your normal temperature by 10 am. At 10 am is when electricity prices start to rise. With the Ecobee it learns when it needs to turn on to get our house to 72 by 10 am. Then from 10 am until usually 5 pm the AC never runs again. I do have the fan set to run 20 minutes an hour to help circulation. Our home is able to retain the temperature fairy well. On the hottest days the AC would turn on around 3.
Copy pasted from my ComEd dashboard:
Total Bills to Date 15
Dollars Saved $277.16
Percentage Saved 23.0%