I've been very excited about Roslyn since I heard Anders Hejlsberg speak about it at Microsoft's Build conference, but I never had a use case for it besides the tutorials and samples included with the CTP. One of the cool things that came with the CTP was the new C# Interactive window (a REPL environment) it added to Visual Studio. With it, I can execute C# against my solution on the fly. This is extremely useful for doing things like testing regexes, comparing the results of different method overloads, or quickly testing a method I just wrote, without requiring me to rebuild the entire project or solution.
The idea behind Compilify is to bring that same interactive environment to the web.