I discovered Horse Agility when my old boy retired from riding and I was looking for a way to make ground work more interesting. Horse Agility turned out to be great fun for both the ponies and me; it gave a structure and a focus to our groundwork that turned work into play.
For the younger, somewhat cheeky, pony it engages with his strong play drive, he doesn't even notice that we're working on obedience and respect (his, for me!) at the same time ;-) As for the old pony (23 when we started), it completely transformed him. His personality was kind and gentle, but very un-confident, he didn't run away but he would freeze when situations got too much for him. Within a couple of months of doing Horse Agility, his confidence soared, he came out of his shell (so to speak), his separation anxiety more or less disappeared and he just became much more relaxed all round. The ponies and I are hooked!