I'd mostly be interested in using a smart thermostat for logging.
If I can detect HVAC performance problems just once before they lead to a dead system on a deadly hot summer day and an emergency call to a repair guy then it would easily have paid for itself in comfort.
I have a Wifi connected thermostat and it has already proved more than useful.
I live in Canada. Went on 2-week vacation to Florida in the middle of winter. Did not check email the first day.
Next day, checked email. Furnace had been sending an email saying "high pressure switch stuck closed" for the last 7 hours. The barrage of emails started at 4:43 am. I had left the key with the neighbours and they were kind enough to let the service person inside the house. The service guy fixed the furnace (under warranty) by replacing the malfunctioning part.
TL;DR: wifi connected thermostat ended up possibly saving the house from frozen, burst pipes.