My 5 yr old mollie mule, which I have had since she was 6 mos old, has been a satisfying challenge. Slick, fast, and damn smart (part of the challenge), training her was not easy. And she does love to poop in the barn (I think she knows it's more work for me). Her abilities are still growing, and she still needs to be less "spooky", but as long as I keep exposing her to new things, it'll get better. She loads, stands still for checking her feet or saddling, is good at tracking goats (cattle soon), and pulls well from the horn.
Seems every picture I've taken from the saddle, her ears are in it...