I've tried the repl shell that the mono guys released some time ago, but it had some major quirks back then.

Compiler as a Service is gonna be huge - think about it: never again powershell with its -eq, -gt syntax and no more painful sharepoint package registration, debug and continue for x64, but best of all an immediate window for fast prototyping like f#, Visual Studio macros in c# ahh, the possibilities are endless...