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.
I tried it one summer but didn't notice any benefit at all from it and so never bought it again. The horse I was giving it to wasn't crazy about it but if I mixed it with sweet feed he would eat it. Reluctantly.
My take on the garlic thing is that you either swear by it or kick it to the curb completely.
I have used it in Florida and never seen any difference at all where we were swarmed by flying insects terribly and would try anything so after two years of trying, it was of no help so no more. However I do think it has its place to ward of vampires because I don't seem to have any of those around any longer.
We used it with one of our horses who has a fly allergy for about a year. Initially the do not like to odor when added to the grain, but they do become used to it.
However, cannot report any significant decrease in the allergy or flies. I even planted garlic all around the dry lot to test if it was effective one year, and cannot report that it made any difference.
Also tried the fly predators and they seemed to help somewhat.
One thing I forgot to mention, it seems to work better if you start them on it before fly season starts, so it builds up in their system. The first year, I ran out before the summer was over, and did notice a difference with the number of flies and how much they stomped their feet.