A couple of years ago we were pestered by flies in the house. I went ahead and bought one of those devices with a blue filament. None of the flies would go near it. In desperation I swatted a fly with a newspaper and while stunned poked it into the metal grill. Guess what? It flew around before exiting unscathed.
They're not like the industrial ones, for sure. The only answer is to wash the cat bowls every time they've finished eating. If you feel like a bit of sport, those 'tennis raquet' fly swatters are great.
Yes, we've got those electric fly killers, and in the house they seem to do no good at all. put them outside on a summer's evening though - and you can hear the mozzies and other nasty things fry!