I haven’t had much reason, until recently, to start messing around with .NET 4 and C# 4. But I just had to do a bunch of stuff involving REST and JSON, and I found that the new dynamic type was very helpful there. Specifically, I used EasyHttp (available on GitHub), which makes it very easy to consume RESTful web services, and (via JsonFx) serialize and de-serialize JSON.
I know that there are other ways to do all this stuff, but I think EasyHttp and JsonFx work well and are easy to use. With dynamic objects, I don’t have to worry about mapping a JSON result to an explicit .NET class; I can just access those members I need to access and ignore the rest.