Most of my writing is done with one app -- Ulysses -- which comes in both OS X (US$44.99) and iPad ($19.99) flavors. You'll notice I said "iPad" and not iOS, which is because The Soulmen, developers of the app, haven't yet released a universal version of the great text editor. Today the company announced that they'll be opening a beta program for the universal version of Ulysses, and will be picking 500 users with "various background and experience" to help test the app.
One feature of the universal app, expected in the first quarter of 2016, that I'm looking forward to is full compatibility with the multitasking features of iOS 9. The upgrade will provide the Split View and Slide Over features of the latest version of Apple's mobile operating system, and will be free to existing customers.
You can sign up here to be considered for the beta test; registration is open until December 14, 2015.