Trello is one of those apps that you can dismiss too fast because you think it doesn't do enough, but which to come back to because everything else is too complex. I've been using it extensively, and it has almost gone viral around me. As in, every time someone needs to manage a project, I hear "Hey, what was this simple project management software you told me you're using for everything you do?", and there comes a new Trello user.
I even remember using it for managing my own time when I was working for a large Japanese company that was using excel spreadsheets to manage projects, so I could have a clear interface and use a quick and dirty piece of JS I wrote to generate the sacrosanct Daily Reports I was asked to provide.