Smoke detectors save lives, period. You should have a smoke detector on each floor
and near sleeping areas at minimum. If your detectors are battery
powered, check the batteries once a month and change them once a year.
If they're nearing 7 years old or more, they're too old to be reliable
and you should replace them!
Carbon monoxide detectors are equally important as the gas is deadly
and you can't smell it. We have several styles including combination
smoke and CO detectors as well as explosive gas detectors.
Ionization technology is best at detecting fast flaming fires.
Photoelectric technology is best at detecting smoky, smoldering fires.
Some interesting links:
Fire Safety Council
Ionization vs Photoelectric