Everyone I know with them ends up turning them off because of the following reasons
1 - Near a kitchen, goes off when you open the oven door or cook a steak. Frigging annoying.
2 - Run out of batteries and chirp and wake you up.
3 - Have loose contacts or something, so despite having a full battery and being hard wired into the house they still chirp randomly (this happened to me)
You could say that people should be more on top of them which is true but any compulsory system like this should take annoyance into consideration. False positives are a failure of the system. It's a shame they're compulsory to some extent because people just get the cheapest ones that are probably next to useless due to the false positives and annoying chirps all the time.