I used it a couple summers in a row for my saddle horses (before I got minis). It seemed to work very well for the biting flies, but the nuisance flies (manure flies) were still buzzing around (although they don't seem to bother the horses that much). My AQHA would break out in oozing bumps on her belly from the gnats before I tried Bug-Off, and she was completely cleared up on the Bug-Off. [i sent them a picture, and they used it in their catalog for a couple summers.] I'm thinking about trying it again this year, as none of the fly-sprays I've tried the last couple years have done a darn thing for keeping my horses fly free and comfortable.
As far as getting them to eat, you have to start out slow, as it does have a pretty strong odor. I found if I wet their feed just a little (like sprinkle a little water on their grain/pellets), the powder would stick to their feed and they would eat it better. My gelding would blow the dry powder out of his feed pan, so the little bit of water really helped.