Makes organizing and re-organizing your writing project significantly easier. It's a no-brainer for the price and I haven't looked back.
The built-in tutorial is comprehensive, but a bit overwhelming. The Windows version is a bit behind the Mac.
I love Scrivener. I'm less than enthused about the Windows version, but on Mac it's fantastic and windows is getting there.. Wish there were better ways to sync data but Dropbox is decent enough. Better integration with citation software would be nice too.