I use my own app to do all of my writing. I like to work a specific way and there were no real writing apps (this was in the early days of iPhone apps) so I wrote my own. My app, Auteureist™, has evolved over time and I am currently in the process of rewriting it. So, no, it's not on the App Store.

Yes, I've tried Scrivener™, but it's not for me. My app does however import and export Scrivener projects.