Small flaws: in an old building with épaix walls, the wireless connection between the thermostat and boiler control box does not pass, will thus require a wire connection. By cons, this wire connection eliminates the USB charger.
Sometimes the activities from the thermostat reports are completely zapped for one day, for no apparent reason.
Very Good: telephone support service nest
What about saving on heating bills? I do not know yet, to see after one year (or at least one winter!)