They are generally considered a good thing. They eat everything as long as it is dead. They can consume meat, dairy, and all the other things you are told not to compost, They like warm and moist compost bins, and will usually disappear with cold weather. They are mainly in covered bins, because in open piles birds will come and make a meal out of them. If you want to get rid of them, add some carbons (straw, paper, cardboard) and turn the bin a lot.
This will cause the bin to heat up and dry out. I personally let them be and do the work nature intended for them. The fly does not bite and it only feeds in the larvae stage. The fly basically lives to lay eggs. They are a good addition to my compost bin.