I'm part of a startup that is all written in ASP.Net MVC and we all really like it. The costs of the licenses were our only initial concern about using it, as they are a bit prohibitive, but it can always be hosted on something like Azure. All our back end systems are written in C# as well. For us, things like the excellent debugging features in VS.Net and the fact that there was a lot more experience with Windows and SQL Server in our team, we decided to keep to our strengths, so as to get going as quickly as possible.