I'm bored and I like showing off my setup so I hope you or anyone else reading doesn't mind picspam:
One of my favorite features: I have my research document with the transcript of the game lines open in the bottom window, and the fiction itself up top. This is better than 2 (or more!) TextEdit windows open.
I do a lot of backstory stuff, as you can see, and I like how I can organize them all in folders and then close what I don't need. Again, if I was using something else, if I wanted to page through my research looking for "which document did I put that one thing in" I'd have to manually open a bunch of rtfs, whereas this way they're all in the same screen.
Smaller screenshot, using a document link:
I can click that and go to the Reach info page.
Basically, I know Scrivener isn't for everyone, but given that I tend to produce a ridiculous amount of backstory, and that I routinely shuffle through all of said backstory in order to write, having it all documented right there in one app in one window is worth purchase price alone.
(Is mildly irritating to keep exporting crap to dropbox for other people, though.)
(Sidenote: waaaah I forgot how much I liked this universe now I want to write in THIS ONE instead of the other two I was working on... Does anyone else have the problem where they have too many universes and can't focus? I think I have genre-ADD.)