I have a nest thermal system and smoke detection system fitted. I've been using this for about 2 years now. Nest is simple to install and once you are happy with the settings, you just leave it alone..
The nest smoke alarms (5 fitted) and thermostat talk to each other and the "away from home" feature on the thermal system uses the sensors on the smoke alarm and such things as knowing when a particular phone is in the home to decide if the hearing needs to turn on or not. Very clever.
Someone mentioned about moving the thermostat around to different rooms to ensure you get the right temperature where you are sat. Nest are just about to release a simple thermostat which you put into other rooms, I think they retail around 20pounds. This communicates with the main thermostat to ensure even temperature throughout the home.
Nest.. highly recommended by me. It works and it's simple.