I have some friends who have a mammoth jack. I know he (Beauford) is very loving and friendly, they haven't had any problems training him except he is a bit lazy and slow but that may just be a individual thing.
They bought him to breed for mules. They aren't sure if he bred any of their mares since its been pasture done. It will be interesting this spring if any foals arrive. I'll be sure to get some photos up if there are any.
I know a mammoth jack like a donkey can breed to a horse and get a mule. A mule can't breed to a horse and get another horse or mule. At least normally, I have known it to take place but its rare.