I'm using Ulysses because you can just type away without the many options word processors have. About all I miss is italics, because this is used widely in books and does have its uses.

There's a solid feel to it -- I find I trust Ulysses to type away into. Another very handy thing, perhaps not mentioned enough, is the Latex export. I'm new to this format myself, but if you get hold of TeXShop to make use of an exported Latex file, then hit typeset and print... well, you get a really, really nice looking manuscript; easier to read than a standard word processor print.

One final thing I'll note -- I had a little look at the saved Ulysses file. It is a package. So I did a "show package contents". Inside are more folders and I found the text documents of my writing. Somehow I found this very reassuring, as text files are simple and can be used on any platform. Also, it showed me that Ulysses is doing some extra work behind the scenes compared to, say, a word processor, which would generally save your work in a single file in its own format.

In short, features/qualities I like:
* Stable feel. * Clean, non-distracting interface. * Latex Exporting. * Clever saved files.