A couple years ago when I was working for a mostly rails shop, We built a medium sized e-commerce system in .NET MVC.
It was our first C# project (though we'd done java before). It was actually quite nice.
.NET MVC is, as some others have mentioned, a rather shameless rip off of Rails-style mvc, but they did a good job, and they published the source, which is handy.
C# is pretty nice, after working with it I actually prefer it to Java.
When we finished the project we were so pleased with the .NET MVC stack that we tried to see how we could make it work for other projects, but it came down to tools and platform:
We had to run windows for development and deployment if we wanted to run Visual Studio and IIS. VS is a decent IDE, but having a *nix system is necessary for literally every other thing we do, which meant we were either running windows in virtualization or dual booting (we have no desire to run linux virtualized for development).
We looked into the mono tools, but trying to get our code running there was an exercise in translation: all the tutorials and documentation are focused on the windows world, so it's difficult to find resources for working with .NET on mono for web development.
In the end, .NET MVC is nice, but it's not such a game-changer that we were willing to put up with the windows-centric toolchain and, just as importantly, the windows-centric culture surrounding it. Unfortunately I can't see Microsoft making the effort to push it to a wider audience.