Yes smart thermostats are great. I live alone and have a somewhat random schedule. Being able to turn on the heat before heading home is a total game changer. If I'm away when I'm usually at home, I can change the schedule in advance, or change it when I already left if I forgot. This helps save money. It can also track usage, so you can double check your energy bill with your actual usage. Although I have a Homeassistant setup with sensors to track usage from the meters, but still a useful tool to have. If you use gas for example for heating and hot water, the thermostat can give the data needed to split up the gas bill between those and see where savings are to be had. It's also an extra temperature and humidity sensor, keeping track of how comfortable your home is and it's possible to act not just on temperature, but other factors as well. I have a bunch of temperature sensors scattered in my home and the curves are useful for tweaking heating and ventilation in the home. Giving an optimal balance between cost and comfort. And preventing things like mold, which might save on heating in the short term, but put on costs in the long term with health issues and mold damage.

I have a lot of automation, but I have one rule. Everything must still basically work when the internet is out or the home automation has issues. So I use physical switches with sensors and relays, when everything fails the lights will still turn on and off with the switch. If there is no internet, physically turning the thermostat up or hitting the big override button next to the heater still turns on the heat. Stuff like that is important, it's a luxery and a convenience, but it must never become a hindrance.

I try to use open source stuff where I can and have contributed to some projects. I've made stuff myself like sensors with self made pcbs in 3D printed enclosures. But I also use some proprietary stuff, like for example the Nest thermostat. I bought it about 10 years ago and mostly because I loved the design. This was when they were recently acquired by Google and were still fully autonomous. Back then there weren't many alternatives and the Nest was by far the best looking one (imho). The software absolutely sucks, the old Nest app didn't get many updates with Google, but the older models still only work in the Nest app. But with Homeassistant I can work around most of it. It's a shame because Nest had so much potential and was doing good stuff, now under Google their product are kinda meh.

Anything recommended by the Open Home Foundation or partners with Home Assistant

I have a Sensi thermostat. Highly recommended.