I don't find myself actually using gradual typing though. Not very often at least. Even in prototypical stages of an application, I usually just put together the types of what I currently have, even if I think they will change later.
Typescript is pretty similar to C# from what I can tell. Not a surprise considering that the designer is the same.
I have found it very useful for things like translating incoming JSON or XML to the typed world. As long as the input data is of known format dynamic code is much shorter and concise.
I don't find myself actually using gradual typing though. Not very often at least. Even in prototypical stages of an application, I usually just put together the types of what I currently have, even if I think they will change later.