Read my story. Really.

Old farmhouse, lots of wood, many rooms, smoke alarms in nearly every one. Originally I got ionisation alarms because I needed a lot and they were quite a bit cheaper. Lately I have been replacing them with photoelectrics, which have come down in price and which I now know are better. 

Computers at one end of house, sitting room at other end. That is where I was just now when I heard an alarm go off. Being cheap ionisation alarms, they often react to humidity or nothing at all so I am used to false alerts. But the alarm kept beeping and my cat was in the room and the cats hate that sound so I went to check on things.

I often have the computers on all day. The first thing I noticed was the acrid smell of burning insulation. Then I saw the smoke coming from the corner where the computer was. Then I saw the flame inside the computer. It was still working normally so I pulled the plug and blew out the fire. At first I wasn't sure of the source, though it seemed to be the DVD drive. But on closer inspection I saw that the SATA power lead was completely disintegrated. I still haven't done a close inspection but it looks like the lead shorted and set the insulation on fire. Maybe the DVD drive is faulty but I don't think so. It will be awhile before I can go over everything.

That was a very close call. Thank goodness for the smoke alarm. It may have 'only' been the ionisation type everyone is now so down on, but it really came through and saved the day. I can't express how grateful I am to it.

Photoelectric alarms may be best, but this experience dramatically demonstrates that any alarm is much, much better than nothing. All of us here surely know better than to be without, so let this serve as another timely reminder for your friends and relatives.