My farrier swears by it. Just a can or bottle poured in their feed every day for a week is supposed to do the trick. I tried it last year but I think I started too early because I didn't see much in the way of results. My one mare just has such a heavy coat it takes a good 2 months to shed out. I was hoping to hurry her along. Maybe now is a good time to try again this year.
Also, shedding is based on hours of daylight, not the temps. Same with when the get their winter coat. When days get shorter, they start growing their winter coats. Of course the thickness of coat is somewhat regulated by temps. I mean, generally, a horse in Florida won't get the same kind of coat in winter that a horse in Minnesota will get.