I have had the oppurtunity to ride a mule. The guy only rode mules, he trained them. They were even used in some western movies =) She very nice and they were all very calm.
I did here though that if they are abused, they are not anywhere near as trusting or forgiving as horses. I have also been around, rode and trained several mammoth donkeys. Donkeys, when you can get them to canter, have the smoothest gaits in the world! I couldn't even feel she was cantering! They reason... alot. lol. And, they can form an incredible bond to you. But I hear that if they have been abused, you can forget it, you can't turn them around.
Lettin' the cat outta the bag is a whole lot easier than puttin' it back. -- Unkown