When Trello came out I had been working on ThetaBoard for a while. Both products draw heavily from some Kanban tools that have been around for years.
If Trello had been out earlier I might not have started my project. That being said, I'd argue that the UX/UI and aesthetics of the two apps are different. I'm going to be making some improvements this year that will differentiate a bit more. As long as people keep using it, I keep plugging away at it.