(Ill write this in English, even if we are on the French store, to help the developers ...)
I like the metaphor of the river for the workflow, and I think the app seeks to solve a real problem for writers, one which has two possible approaches: one is to throw in the kitchen sink and develop ugly bloatware (think MS word.) ; the second is to carefully design the app from the ground up, adding features only after careful consideration (think Vesper.)
I think Writer Pro is taking the second path, which puts it in a special place, with other thoughtful, well crafted tools: tinderbox, vesper, iThoughts ….
These tools are not for everyone: if they dont speak to you, dont invest in them. They are costly - in time and money, and they require faith that the developer will be able to deliver.
Yet the rewards are in equal measure. The quality of the work done, of course, but also less tangible things like lower stress, smoother workflows, and mostly - discovery (of new connections, new thoughts, new uses)
Is Writer Pro a great app? Its currently missing a few features I need : organizing / filtering, multiple languages, maybe internal linking, probably Dropbox and TextExpander ... So in that respect its not perfect. But Im happy to shell 20 bucks on the promise that the developers will make it better: their previous app was well received, their intro to WriterPro shows deliberation and thoughtfulness - and thats more than can be said for many others.
WriterPro is something better than a great app: its the promise of a great app.