I would be concerned about power outages, surges and the like. If the personal schedules of everyone at home varies randomly day to day, like mine does, it seems unlikely that a "learning" thermostat would help me.
If your bill went down more than the norm for given outdoor temperatures are concerned, then the only thing that matters is whether the cash flow for the expense of the system easily balances out the savings.
I would be less likely to change if the system took a long time to break even with the savings it created. It's kind of like when a home owner plants a tree. The person who really benefits from it is the NEXT home owner. I feel it is that way with a lot of home improvements also.
I have a programmed thermostat that turns the heat down to 59 F during the night and during daytime when I am not there. (We live in hills in Central Coastal CA, so this isn't extreme), so it only comes on for an hour in the morning and a few hours in the evening. The dog sleeps and spends a lot of time directly underneath it, so this could complicate motion detection. Would we benefit in any way from having this? I suspect the answer is no, but wonder if I am overlooking something. I do like the idea.