We switched over in our shop on release week, and I must say that I'm enjoying the new features of .NET and C# 4.0. COM interop with Office Communicator is cleaner. IronRuby + Sass for css with ASP MVC rocks. Optional parameters have killed off many pass-through overloads. Also, DynamicObject and IDynamicMetadataProvider give potential for the dynamic keyword goes far beyond ExpandoObject. It opens up possibility for new DSLs within C#. Although likely to be misused, having the power to manipulate the meaning of the language is an impressive feature.