Since launching Hive for heating remote control, British Gas have rolled out quite a lot of other products which work in the same app and use the same router-connected hub to talk to their end. They all feel quite pricey for what they are, but I decided to try the door/window sensor. Here it is attached to my front door with the supplied sticky pads:
At the moment, it's limited to doing push notifications to your phone when the door opens and/or closes; you can specify time ranges within which you do/don't want the alerts. It's interesting to see a log of when I leave for work and get home, and given that I'm not the only one with keys to North HQ (I have cleaners), it's useful to see any other comings and goings.
The alerts are usually (but not always) instant, and show up nicely on my Android Wear watch too.