Works if it works... Just been to a hotel where they didn't have the "Do Not Disturb" signs but rather an electronic sign outside the door.

Was all connected to the key so you could see on the outside if a) there was a key in the room (nope not just any card, it had to be the end of that particular key) b) you didn't want to be disturbed (a button you had to press in the central command stand on the bed-side table).

Also, often key and light is connected you if you leave your room the lights go off...