I've used Celtx on and off for a while, and my son - a budding film director - swears by it, because it's optimised for people like him.

But as a writer, I much prefer Scrivener. Or MyInfo, or Smereka TreeProjects.

Once Scrivener has been transported to iOS (currently in process), my life will be complete. Well, of course it won't, otherwise I wouldn't be a CRIMPer, but I can dream...