I use it when mine are in hard work/competing. None of them have ever hotted up but I think it does give them just that little bit more energy and oomph that we need.
I currently only have one on it - she's a 4 yr old 13.3 pony that's just doing a few more weeks before we turn her away for the winter. She went on it for PC camp at the beginning of the summer and did so well on it that she's stayed on it. No heating, but just enough sparkle...... When we turn her away she'll be living on grass alone, so I'm not generally one to overfeed - lol!
It's now called Competition Concentrate. I use it on my dressage Welsh D and hunting cob................works fantastically as they need the sparkle without the calories. Also only have to feed a small amount. I also feed them D&H Staypower Museli with it.