Scrivener is one of those app purchases that I highly value. I have written two screenplays on the Windows version of the app and though using First Draft is the industry standard, I like the ease of use of Scrivener. A lot of the features (even the corkboard) seem harder to grasp and I don't use them, but for playing around with chapters/sections and maintaining focus, it's one of the best writing apps I have used.

(Other favorite is Zettlr, but that's another story)