I recently started using a new application for most my writing. Ulysses is a distraction free text editor with a wide range of additional features and an outstanding interface. It's available for Mac and iOS. I was planning on writing a long article about Ulysses, but the folks over at the Sweet Setup beat me to it. They've got a great review of the app.
Ulysses is a Markdown app. (If you aren't familiar with Markdown, it's a way of formatting documents quickly and efficiently.) I'm still uncovering all of the features in Ulysses, but I'm impressed. It's not cheap, but it's worth every penny.
I did have one problem with Ulysses and syncing. When I first bought the iPad app, some text it had written on my MacBook Pro wouldn't sync. I struggled with the issue for a little while, but then figured out that if I made any change to the text, it would then upload and sync. So I added a space to the beginning of each problem file and everything synced perfectly from that point on.
They also mention a few other writing apps as honorable mentions at the bottom of the article. And they are some of my favorites, too. Specifically:
Scrivner: I really love Scrivner. It's exceptional powerful, but really complicated. Lack of sync is challenging and I prefer the Ulysses interface, but for really long form work, it might be the best option in the App Store.
Byword: Still one of my favorite distraction free writing apps. Byword works on iPhone, iPad and the Mac. Great app and I highly recommend it if you want a Markdown editor that isn't as expensive. My go-to note taking app.