Excellent but very expensive. A home made fly trap filled with sprite/7up with a dash of fairy works just as well.
Do you put it in a jam jar or is it more complicated than that? 
If I could sus out how to upload a pic I would show you but I am dim, so here is a description. Take 1 empty plastic bottle, cut around it 1/3 down and turn the opening end inside of the lower part. To create a funnel down towards the liquid. Staple/duct tape together. There are loads of videos on YouTube showing how it's done... probably better than how I explained it.
The flies seem to find the sweet sickly 7up or sprite soda drinks delicious. I suppose the downside is that the bees also love this drink.