I played with Scrivener during November, and had decidedly mixed feelings about it. So I decided I should give the other main IDEs-for-prose a shot! My feelings about them were decidedly un-mixed.
Ulysses 2.0: I tried the first version back in '06 or so and loathed it, but my tastes have changed, and I figured its focus on markup-based* formatting might appeal more to me than Scrivener's NSTextView WYSIWYG. Yeah, no. God no. This program runs so counter to the way my brain works that I actually have a hard time imagining the existence of people who find it useful.** It has all the failings of an integrated writing environment, but it consciously rejects all of the benefits. I absolutely remember why I threw it aside last time and just went with BBEdit and a folder fulla text files.
StoryMill: Also no dice; even reading through the tutorial file made my head kind of spin. The dealkiller here was how much fidgeting and toggling and administrative nonsense you have to go through to do anything. Are you typing in the "Notes," or the "Text," or the "Chapter?" It's all the same text field, and you have to constantly fiddle with these controls at the top to make sure you're in the right place, which, NO, I have better things to do with my attention. (Also! It failed at importing some UTF8 text files and spat mojibake throughout. WTF? This is 2009, yo, I should not need to babysit charset conversion.)
In the meantime, it turned out that Scrivener'd made a good enough case for its utility during the trial period that I did, in fact, miss it. I have issues with it, but it's... actually a pretty good implementation of something that will make part of what I do a whole lot easier. And it fits with my current write-in-bits-on-the-train thing really really well. So, I bought it.
* Just for the record, what bugs me most about WYSIWYG composition is modal emphasis with no indicator. As in, you're looking at the screen and there is a blinking cursor: when you type something, is it going to be bold or italic? WHO KNOWS. Drives me bats. (Brief props: MS Word slants the cursor when you're in itals, which is cool. Everyone should do that, and make it blocky when you're in bold. That would solve the problem perfectly. But I seem to be the only person who's infuriated by the current situation, so whatever. Grumph.)
** Obviously they're around, and I am glad they enjoy it. Space aliens deserve good software too!