I have been using Scrivener for a year or so now. I love being able to see things on a sort of cork board, and see summaries of each chapter. It can be tricky to work out how to use it, but definitely worth it. Although once I compile it, I edit in Word and format in Word.
I've used dozens of types of software. Hope springs eternal that it will improve my writing.
Word (still use for formatting and editing)
Scrivener (have used it for 9 years now--3 were in beta) and love it, but still, am not using all of its power.
CharacterWriter (for character development using DSM IV and plotting help)
Liquid Story Binder (a friend recommended this because it combines music listening to the story, but I never took to it)
One Note (I use this as an aid to writing --bringing elements of my story into One Note to keep all the odd things in one place like pictures, drawings, sketches, maps etc.)
Dramatica Pro (I got bogged down in their outlining system. I think it could have worked better had I devoted more learning it)
Final Draft (mostly for writing screenplays but also for novels)
Y-writer (free and fairly good, but Scrivener is better; Spacejock guy has other good programs for tracking writing and submissions--also free)
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.