I too saw no difference with the Smart Bug-Off, but I have used garlic from www.springtimeinc.com and have noticed a huge difference in the flies and TICKS. I kept my gelding at a friend's house and her place is a perfect place for ticks, in fact, she was pulling them off her horses daily. My gelding had ONE tick the entire summer, and it was on his nostril. The garlic seems to keep flies away from the heavier muscled sections of the body, the face flies still seem to come around, but the body not so much.
"If you think nobody cares about you, try missing a couple payments..."
There are two types of "feed through" fly products. The ones like Bug Off and Buggzo are supposed to repel flies. Useless, IME.
The other type (Solitude, Simplifly, Equitrol II) kill fly larvae in the manure. Very useful, IME, but ALL horses on the property need to get it and you need to start feeding it weeks and weeks before the flies come out. Mine have been on it two weeks already.
One of mine was really having a problem with mosquitos at a previous barn, so I put him on Horsetech's Buggzo. I did feel it helped as he was less irritated, had fewer bites, etc. compared to other horses near him. He also wore fly sheets almost all the time and was getting Mosquito Halt applied often. So can't say for sure the Buggzo was the primary deterrent for the bugs, but it did seem to be a help. It has a very strong odor that I thought would turn him off (super picky at times), but he eats it right up.
We have a horse that has a skin reaction to flies, so we tried the Smart BugOff. While I didn't notice a difference in the numbers of flies, her skin returned to normal. I think that it just helped her deal with the flies that were around.