I have just started using this stuff and I've got to say, it works much better than many of the commercial fly sprays (not to mention it's a heck of a lot cheaper). Since it's a concentrate, you can mix it a bit stronger if the flies are thick or weaker if your horse is sensitive or you just have a few flies. The only thing it doesn't have that others do is the citronella scent. I'm not sure if you could mix a bit of citronella oil in with it without a reaction or not. Now that I'm thinking of that, I may look around to see if you can.
Just think, mixing it as strong as the instructions recommend, one $17 quart will make 25 gallons of spray. Permectrin II KMG Chemicals (Equine - Fly Control - Fly Control Concentrates)