Wouldn't that more or less be the Immediate Window that Visual Studio already has? Other than the multiple lines you mention. :)
I took a look at this to see if it can replace LinqPad, which is similar. LinqPad allows for more than C# alone (VB.net, SQL, C# and F#) but it has no code completion (unless you buy it) which CShell does have.
I highly recommend buying a LINQPad licence. I use it several times a day now and it's so much better with the extra features.
My favourite feature is the ability to point it at a DLL which has an entity data model in it - and then work with that model 'live' as if you were inside app code.
Normally, to do that, you'd have to put a breakpoint in your app and use VS's Immediate window - which doesn't support useful stuff like lambdas.
I took a look at this to see if it can replace LinqPad, which is similar. LinqPad allows for more than C# alone (VB.net, SQL, C# and F#) but it has no code completion (unless you buy it) which CShell does have.
This seems like a useful tool!