I use Scrivener and really like it. There are a lot of bells and whistles in the program that I don't know how to use (I'm probably missing out on something awesome but I just can't be bothered, lol), but all I really want out of Scrivener is a good word processor. I love that I can use it for storyboarding as I'm planning my outlines - I can write each scene on a separate "index card" and pin them to the corkboard, color code them, drag and drop them anywhere I want to change their order in the timeline, etc, That feature is incredibly helpful to me because I'm a very visual person. It's also great that Scrivener has every single file in my book available at a moment's notice (as opposed to Word, which takes - on my computer, at least - several seconds to open a whole new window).
So I am a Scrivener devotee, but this Ulysses program some are talking about sounds interesting, as well. I'm going to check into it.