Jim,
If you already have a decent alarm system in your house, it is usually very easy to connect up fire, smoke and carbon monoxide sensors to it.
No need to worry about changing batteries, or not being woken by the faint shrill of something coming from the garage. They give 24 hour protection, whether the alarm is armed or not, and if a problem does occur, the whole system goes into loud screaming mode.
They are a lot more expensive than the battery ones (about 4 to 5 times the cost), but to my mind, well worth it.
John