1) The thermostat drives most home energy usage. It's probably above $1000/yr for most households.

2) The existing programmable thermostats were awful and no one used them effectively. There's a ton of waste as a result.

3) The smart thermostat can easily pay for itself in a year or two just by having a better UX that you'll actually use. That's an amazing value proposition.

- it didn't look crap;

- I could figure out how to work it

I'm not so bothered about it being smart, least of all for it's 'learning', it just looks so much better on the wall (visually prominent when you walk in) than the white plastic sharp cornered box the developers put up.

If I spent more time/energy on it I'd do away with a wall-mounted thermostat entirely, and just have a HomeAssistant-connected relay right at the boiler.

Granted our thermostat isn't really "smart" it just has an API exposed that integrates into Home Assistant.

I wouldn't put in one of those cloud based ones like Nest or Ecobee