We used them last year because my cousin was going to have his wedding at the farm and we didn't want guests to be inundated with flies. Let me disclaim that we had 6 horses at the time (2 minis) plus 8 goats plus chickens and the neighbor has cows with a large pond. It is important to get the proper amount of the predators--a few won't be enough if you have tons of animals. Also with chickens we tried to keep them up a few days after releasing the predators so they wouldn't eat them.
They did work well, but I noticed a big improvement after I started dumping the manure far away from the barn instead of right outside. Flies will go where the poop is so the further away the better. Also its good to make sure you aren't leaving out "dirty" feed dishes and other attractants. And only use fly tape in the barn, the other smelly stuff will just attract them into the barn.
I would definitely recommend them for anyone with a "normal" or horse only setup. If they could work for us with that many animals and issues it could probably work for you.