Beautiful, distraction-free writing environment. Thoughtful design. Works especially well on OS X and iOS.
Windows/Android versions are a little underwhelming.
It's definitely my preferred writing environment, from the standpoint of design and feel. Actually, my absolute favorite environment is iA Writer on iOS (iPad Pro) with an Apple Bluetooth keyboard. It's like a modern typewriter, but so much better. Ever since adopting iA Writer, I do all my writing in Markdown and then port it to other tools later. However, I still find myself using SimpleNote, even though it's "uglier", just because all the x-platform apps across Windows, Android, iOS, desktop, and web all work so perfectly. For iA Writer syncs, I use a single Dropbox folder with all my Markdown files. On OS X, this is perfect, but on mobile this is sometimes tricky to navigate. I really enjoy the iA Writer 5 improvements on OS X, though!