Please don't actually do this - or if you do this, use the C# code only for the backend, and write the UI code natively for each platform. Nothing is worse than a 'not quire right' 'seems almost the same but isnt' UI.
You have misunderstood. iOS and Android apps built using C# have access to 100% of the native API of the underlying platform. So all of the UI is fully native. Take a look at our API documentation on iOS, for example:
If you have read the article, you would have known Xamarin products expose native iOS and Android APIs. MonoTouch apps feel exactly the same as UIKit apps because they are UIKit apps.
From my perspective, MonoTouch UIKit bindings are more convenient than original Objective C library because C# is nicer (think events, typed arrays and dictionaries, etc.)
One of the major advantages of Xamarin's approach is that the Xamarin frameworks expose access to all of the native APIs and standard user interface controls of the underlying platform. The apps are fully native, they are just written with C#.
As I understand it the mobile SDKs Xamarin releases use the native UI widgets for each platform - allowing you to get native look and feel with one code base.
(I may be wrong on this, but I thought this was one of their major selling points. However, never having used the SDKs I don't know how well it works in practice)
