I have ridden/helped train a grand total of 3 mules (one draft and 2 QH size). Once they are trained, they are some of the most trustworthy and reliable mounts to be found. Training them, however, can be quite an ordeal unless you've got a ton of experience with them.
If you want a mule, go out and find one that's broke. Once they get to that point, they are really no different than riding a horse as far as a willingness standpoint. Of course, saddle fitting is an issue because most mules have a donkey's flat back and a regular horse saddle will slip and slide all over the place, causing the need for a britchin or a crupper and sometimes a breast collar too.
Be warned though, a good riding mule will often cost 2-3 times as much as his equine counterpart with identical training/temperament, especially if mules are not common to your area.