So I finally swayed for a first trio of Homekit gadgets: an Elgato Eve Energy (smart electricity plug), home (temperature, humidity and air quality sensor), and door sensor. I installed it all while I was home alone, the Eve Energy plug is connected to a big standing lamp.
When I had dinner with my wife, I asked "isn't it a bit dark here", and then spoke in my Apple watch "Siri switch on the living room light", which indeed happened after 2-3 seconds. How we laughed! It is truly like magic the first time it happens. I could also inform about the air quality, room temperature and humidity on my watch.
Anyway, another fun use of the Apple watch. I am also eyeing the Philips Hue lights and will likely buy the Withings Home internet camera (will be a baby monitor) once it is also officially homekit ready - this last one has a watch app that gives a (slow) video feed of the camera.
All in all these homekit gadgets are fun ways to extend the usefulness of your apple watch.
A note: currently those 3 gadgets only work locally via bluetooth when I am at home, I will do a setup via apple tv to make them usable from elsewhere.