I've been using it on my Mac, iPhone and iPad since it launched. I was using iA Writer previously. I'm a big fan of both applications. Writer Pro is quite nice, but it really doesn't strike me as a big improvement over the previous iteration of the product. (What's worse, it lost a few useful features like Dropbox support on iOS.) Some of the gestures on the iOS application are also a bit annoying. I accidentally swipe between documents enough that I'd rather be unable to do so. Demarcating your writing into their various modes is handy, but it still feels lacking in its current incarnation. (The Read mode doesn't render your markdown?)