A mule is by far my favorite trail animal. They are a hundred times smarter than a horse and don't put themselves (or you) into danger. Ever. Another good thing is that they rarely spook or bolt, and, if they do, they are looking where they are going and not running blinding. In my experience, even the jumpiest mule will only run about 50 yards from what scared it before stopping completely and investigating. Most don't even go that far.
They are great all-a-rounders too. Nothing is more versatile then a mule. I've seen some that enter every invent in shows: English pleasure, jumping, a little dressage, Western, reining, cow work... The list goes on.