Living as we do close to a farm, flies are a real problem. A few years ago we bought a couple of those electronic "fly killers", the kind with violet lights and an electrified grid. They were pretty expensive, and totally useless. I tried the tennis racket things but never really got on with them. They don't work if you just swat a fly with them, as mentioned earlier you've got to hold them over the fly & wait for it to commit seppuku. We bought a couple of excellent, large fly swats in Florida, the kind with tweezers to pick the corpses out of the mesh. Very good, satisfying sport.