Mark 5:1-20 - Not about pigs
So there's a couple of things that people can get hung up on in this story and they're worth mentioning briefly before we get to the main point.
People get hung up on the idea of demons and demon-possession and there's plenty the Bible teaches us about them, which we'll leave for another time. In this passage, what we learn is that they're busy destroying a life and that they are no match for Jesus. He's not just in charge over sickness and nature - he's in charge over demons too and they have to do whatever he tells them. They're scared of him because they oppose him and they know who he is (v7).
People also get hung up on the fact that cute little piggies die in this story but they miss the point. As much as we love pigs, they're not as important as people. A man's life is saved in this story (that's the point) and to Jesus a man's life is worth many pigs (even cute ones).
So the main point then!
The facts before meeting Jesus: The man lived among the dead (v3), broke every restraint - even chains and irons (v3-4), fought off everyone who tried to help him (v4), spent night and day in the hills wracked with grief and cutting himself (v5).
The facts after meeting Jesus: The man sat still and calm (v15), wore clothes again (v15), was in his right mind (v15), wanted to follow Jesus (v18) and ended up amazing everyone by telling his story (v20).
It was such a changed life that people were shocked and even afraid at first (v15). These people had all sorts of ideas about who they thought Jesus was but they found it impossible to dispute the changed life that confronted them.
Another story of a changed life - Brian 'Head' Welch (ex-Korn guitarist)