When you actually hit a fly with it, it works amazing - the fly burns off in seconds, however, to get a fly with it is quite a chore. It almost seems that using a towel "swooshing" technique is more efficient or.....
I know, the anticipation of the "or"...... or you can take a ziplock bag, fill it with water and put 4 pennies in it and hang it in a room or a patio. Yes, I know, you don't believe me. Well go ahead and try it!
Another thing that you can try is basil. Flies hate the smell of basil. To discourage them, place pots of basil at doorways and windowsills and on the kitchen counter. Don't have enough sun for potted basil? Put dried basil in a small muslin pouch, rubbing it occasionally to keep the scent strong.
Happy fly hunting!