I do plan on purchasing one of these at some stage to try it out with long reins on a pony.
From what I read, horses/ponies take to it well. But I don't actually personally know anyone that is using one of these so I guess I'll have to wait to try it out myself.
When I do, I'll let you know. But if you get to try it out first, let me know how you get on - I'd be very interested!