I have an ecobee thermostat that I manage locally over WiFi using the HomeKit integration, but I’d stop short of recommending it to new users.

1. Ecobee used to support developer access to their cloud API for controlling the thermostat and collecting efficiency data, but stopped issuing new API access tokens in the last couple of years. They have no plans currently to reopen developer access. If you have a token then the ecobee integration works fine, but if you don’t you’re stuck with HomeKit.
2. The thermostat requires 24V from the furnace to run the display and wifi stack. They provide an adapter you can install if you have available free leads at bother ends of the thermostat control cable. I had to splice a new wire onto the 24V transformer in my furnace since it didn’t have a 24V common terminal on the control block. It wasn’t hard to do in the end, but it was a lot of research.
3. Some advanced thermostat features require the app. I am not sure whether the app uses cloud or local control when on the same WiFi.
4. Not all features are available through the HomeKit integration. I can change the thermostat mode among Auto/Heat/Cool/Off, manage the blower fan mode and manage the heat/cool set points.
5. Data logging. The damned thing does log activity back home, and the data is only available in the app or on thr web portal.

Other than that, Mrs Lincoln, how was the show? I haven’t been unhappy with the ecobee. The HomeKit integration works fine, and I get enough data from the native HA history to track and manage my energy demand. I shied away from Honeywell because my last Honeywell thermostat-- the one I used just before the thermostat I replaced with the ecobee-- tended to cycle my furnace too fast during cold snaps, and it would put the system into thermal protect mode. There was no way to widen the hysteresis (or modify the duty cycle) except by manually setting the temp high, run the house up to that temp, and then lower the setpoint and let the house take longer to cool.

ETA: the ecobee a decent thermostat and I’m happy enough with it overall. It has “spousal approval” accreditation as well. I wish it checked more boxes for me*, but it was essentially free through a power utility program. Its a worthy upgrade for me, but YMMV.

* namely, Z* protocol local control and continued cloud API access

I got a cheap Nest E thermostat on eBay from a charity shop seller. It cost me like £15.

Replaced my dumb one with it, then hooked it into HA and made a sensor from all my motion sensors and door sensors which all have temperature gauges in them.

Conglomerated those sensors into one that tracks the average (making sure to only have so many per floor so as not to skew the data to one part of the house) then made a sensor called “Is it cold enough to have the Heating on?” which acts as a simple switch with a lower and upper limit.

Now my heating only comes on when I need it to based off the temperature of the whole house instead of that one place the dumb thermostat was based.

The display unit for it died after less than a year, replaced that for another £20 on eBay and synced the new unit to the Thermostat on the wall.

So for less than £50 and some smarts I’ve upgraded my heating, and saved money on the bills (since it only comes on when we need it to rather than based on the temperature of one location).

I’m not particularly recommending Nest since I have no experience with other manufacturers, but I managed to do it all on the cheap and I’m quite pleased with the results