Ruby has been gaining a lot of popularity lately (if not even months/year ago) but as I can't even get myself full of .NET I never really given it some look until now.
It looks like the advantage of Ruby over other languages is [more]the expression of the developer's intent along with shorter code. It seems that this is a common of dynamic vs static language (in terms of type checking – dynamic @runtime, static @compile time).
These are fairly old posts but as I've said, .NET has more than enough to keep us busy. Nevertheless, here are some interesting points on this
Programmer Intent or What you're not getting about Ruby and why it's the tits by Scott Hanselman
Channeling Ruby in C# – C# equivalents for some common scenario where Ruby is perceived to be better by Derek Slager
Looks like C# is catching up with it's new features but then again if I have a chance then I would definitely learn another language (Ruby sounds nice). I'd have to agree, learning a new language is fun (that is if you can find time for it).