We have them in our house, Tokyo Gas, or local natural gas utility company also insists on it, they do CO2, and natural gas, etc. They work, they work really well. When we first moved into this place, there was a minor problem with the gas oven, sometimes it would light for a few seconds, then the flame would go out, and gas unburnt gas would still come out for only a second or two, then it would shut down, and we got an error message. The first time this happened the detector went off, scared the heck out of us
We could NOT smell any gas, and usually you can smell even a small leak. The gas company guys came, it turned out to be a problem with the sensor on the gas oven, they fixed it and everything was OK.
The second time, we had a BBQ party at the house, and we were using a portable gas hotplate thing, in the living room for something (I forget what). It was a hot day, we had 50+ people in the house and we had all doors and windows open, it was also a bit windy (6 floor up you know). Anyway, we figure the hotplate thing got blown out by the wind, and the detector went off again, so in a house full of people, all the windows open and no one smelled gas, but the detector went off.
I will continue to have the detectors in my house.