I thought Unbreakable was a great movie, a superhero story unlike any other. Unlike other superhero movies which are clearly set in fantasy worlds, the realistic style of Unbreakable made you believe that it was all really possible. The character of David Dunn was just a regular guy with a failing marriage and a dull job, a realistic person that you could pass on the street any day of the week.
I think it might be better if it was left as a stand alone piece. Any sequels might make it become a franchise... and then it would just seem like a regular superhero movie.