Be careful to get a real, properly functioning one, though - there are quite a few which just cash in on people's fears without being terribly useful. Check it's got an official certification or something (I went out with a gas boiler maintenance person for a while and feel duty bound to pass the message on...)
We used to have a carbon monoxide detector, but one day this summer there was like a magnetic disturbance in our house and our door bells started going off all the time and the CO detector would beep oddly (not in a way that there was a problem with CO, just to beep for the heck of it) so we dismanted the door bells and unplugged/removed battery from the detector.