We've written a custom compiler from C# to Java and JavaScript based on Roslyn. Over time it gained more features and target languages as well (Python is in progress, we can also emit a working GWT wrapper for the JavaScript output, we can emit TypeScript typings or just normal TypeScript as well, etc.). For us this helps us in offering our products on various different platforms without having to write the code in different places anew. Since our product is a library, not an application, we couldn't really take advantage of existing conversion tools that mostly take the path of converting IL to hideous code and an entry point.
The whole thing is now used in basically every library build we have at some point, even for the C# versions, as it ties in with our documentation writing process and places the correct API names and links for that product into the documentation, even though the docs start with mostly the same content for each.
I agree that lack of documentation makes working with Roslyn a bit daunting at times, although the API is very well designed and oftentimes it's very obvious where to look for something. I was also very impressed by their compatibility efforts. We started while Roslyn was in beta and upgrading through the releases worked without a hitch.