I used them when I lived in Nanjemoy (had my horses in my backyard - not many horses all that close) and they were GREAT - not one stinking ankle-biting &^%$ stable fly. They don't do anything for deerflies or horseflies, though - keep that in mind.
Now that I'm on Dodon Farm we're doing it here, too. I started it last year and I think I should have gotten a double-order for the whole season. Doing it again this year with larger numbers. Though we did see a reduction in flies, it wasn't like I had with just 3 horses in the backyard. It cost me $600 from Spalding Labs for the whole season (double batch in June or July - can't remember which).
Though if it'll save on fly spray....