Having an interactive code interpreter is a huge productivity boost. It allows you to easily run and test your code as you write and modify it while not needing to pay the dreaded compilation tax which can disrupt your development flow. This dramatically tightens and shortens the feedback loop on how well your code works ranging from whether it is behaving as intended for meeting spec requirements to much more quickly identifying any runtime bugs than you would using a development process common to traditionally compiled static languages. (As an aside, these are generally the same reasons that are given for creating and maintaining automated unit tests. Same goal but different methods.)
Not sure how well this C#/Clojure/IKVM.NET approach works or how well it realistically performs (typically, interpreted languages are slower). What is certain is that this unusual implementation requires the use of the very foreign-looking Lisp parentheses. I will openly admit as someone who does not program in Lisp it strikes me as kind of strange to use and see parentheses with C# but aside from this peculiar syntactical idiosyncrasy the general concept of REPL with C# overshadows even this oddity. This quote sums up its overall appeal in the world of C#:
A REPL is a Read-Eval-Print Loop, which is a fancy way of saying "an interactive programming command line". It's like the immediate window in the Visual Studio debugger on steroids, and its absence is one of the increasing number of things that makes C# painful to use as I gain proficiency in more advanced languages.
This is precisely how I felt when I initially started to learn and use Python. Suddenly, coding in C# with Visual Studio certainly seems to now be relatively more restrictive. Similarly, whenever I have had to touch any VBA code (yes, that does happen from time to time) I customarily inhabit the VB Editor's Immediate Window (IW) pushing its limits by attempting to use it in a manner that is similar to how I code in Python.
For example, in the VB language, not only is it not required to declare the data type of a variable but it is even unnecessary to explicitly declare the variables themselves (usually this is done using the 'Dim' keyword but this can be avoided by quite simply not including the 'Option Explicit' statement). Subsequently, the first time a value is assigned to a variable, the variable will automatically and implicitly be defined on the stack just like it does in Python. As a result, you can somewhat attain that same level of interaction with code in VB (via IW) as you would in Python (via its standard interpreter) potentially gaining the productivity benefits of writing less code in contrast to strongly typed languages.
My increased reliance of the Immediate Window also extends to Visual Studio when coding in C# but it requires more work and syntax overhead versus IW in the old VB Editor. Overall, it is not quite the same experience as in Python. Regardless, as I have previously written, frequent use of the VB Editor's IW led me to lean heavily on the one in Visual Studio whenever coding in C#. Prior to that, I had somewhat forgotten it even existed. In fact, in VS 2005, the IW sometimes is missing and difficult to view when not in debug mode (this is allegedly also true for VS 2008). This is discouraging as it probably contributes to most .NET developers not favoring its use in more situations.
While I have seen other attempts at providing an interactive console for C# the following are ones I have noted to possibly try out in the very near future:
more dynamic C# 4.0 becomes mainstream. (How long before we have an official C#Script? It worked for VB and VBScript.)