we have them in England but are not the law, theres been a big campaign on them by the fire brigade and have handed out fire alarms by the thousands (i have mine free and top quality)they cost around a fiver (£5.00) including battery the one off the fire brigade cost around £15.00 each, the theory is the cost of saving someones life by being called out, is outwayed by the cost of the smoke detector.
The problem they have is when someone is cooking and the alarm goes off people take the battery out to stop it shreeking, and dont put it back hence they then have no smoke alarm.