I've been working with it and it's not bad - won't tear my hair out if it's what I use in the end. But a couple pain points for me are the need to rewrite my code to grab 'session variables', needing to understand how images are returned, especially if you need to return more than one, and so on - versus Shiny, which allows me to more or less drop in the code I've already been working with. Oh, and parallel requests are just not possible.

Of course, most of these issues are understandable and possibly by design, especially since OpenCPU was designed for embedded systems. That's fine - it just makes creating webapps or dashboards around R a more complicated usecase.