I have been trying out Ulysses for writing the last couple of days. It is a pleasure to use, and I quite like the interface concept of sheets and groups rather than files and folders.
What I don't like about it is that, despite the fact that it is basically just a text editor, all the stuff I write in it is buried deep in some library folder, obscured by inscrutable alpha-numeric file names. If I ever want to get at them via any other app, it's anybody's guess where any given bit of text might be buried in this morass.
I tend to prefer the Notational Velocity approach, wherein you just point the app to a directory filled with text files, and it sorts the whole thing out internally, leaving all the files intact and available to any other editor you want to use.
Ulysses is still a nice enough writing experience in spite of that drawback that I will continue to use it. I need to dig more into its "external folders" feature, which may help to alleviate my concerns.