My mom-in-law swears by it, so I bought some and hated it. The flies and ticks were just as bad as before and it made the barn REEK like garlic the entire summer. It was gross.
I have heard that a small amount of apple cider vinegar will acidify the blood just a small amount and drive the flies off. Haven't tried it tho.
I personally hate drenching my horse in chemicals and refuse to do it. I found a great all natural product that I adore. It is called belly balm and is a green ointment that smells like citronella and has aloe in it. I rub it on her belly, by her eyes and behind her ears and never had a fly problem all season. Our gelding gets horrible hives and sores from flies and we used that in the areas not covered by his fly sheet and he healed everything up that was there and never got another sore. I would definitely recommend it!