I tried them for a year, but the thing to remember is that you are treating the land, not so much the horse. My problems were due to the large number of cattle in close proximity to my horse paddocks. To have the bugs be effective, I would have had to purchase the quantity for 25+ horses and treat the cows as well (not nearly as cheap as treating for 2 horses!). Maybe a good (and economical) situation if you have 5 or fewer horses and have the only livestock for a fair distance, but they didn't do a great deal of good for my situation.
I have a few of the reusable jar fly traps. I fill them halfway with water, lunchmeat, and a horse turd or 2. Half a day and the dead flies in it just add to the stink and attract the rest. Just be sure to put it a long way from the house.