I have published a CodeProject article documenting NRefactory 5: Using NRefactory for analyzing C# code
While NRefactory 5 itself is pretty much stable, there's still a lot of work to do for SharpDevelop 5 to use it -- pretty much everything in SharpDevelop dealing with source code needs to be ported to the new NRefactory 5 API.
But I think this article shows that NRefactory 5 will open up a lot of new feature possibilities. For example, we could detect problems like the missing StringComparison described in the article directly in the SharpDevelop editor while you're typing, no need to recompile. In fact, the MonoDevelop project has two GSoC students working on such live issue detection and other code actions (small refactorings), and most of their code will be usable in both IDEs.
So you can look forward to a much improved C# editing experience in SharpDevelop 5 :)