I know some people like to work bison because they stay fresh, but I'd be too chicken.
One of my high school teachers was badly gored by one of his bison. He nearly died. He was on foot, but I wouldn't feel too safe on a horse, either.
I've also seen a bison bull that was very tame. The owners brought it to ag shows all around Alberta, and even into their house. But that guy was raised from a calf, and somehow decided humans were his buddies. I never trusted him, though, either. I don't think I even pet him, despite multiple opportunities.
I am a little paranoid about bison, but I've met at least two people who have been gored. They scare me more than black bears, and I sure wouldn't approach a bear on horseback!