MVC framework for ASP.Net

The MVC framework is quite a different way of building Web interfaces than Web Forms.It’s much more modular and now we can easily test the individual components of the Web application using our own unit test framework.

The feature I am trying to understand the following 

1. How does MVC support in unit testing? Want to try an example to see this working

2. How extensible and customizable MVC really is?

3. Should do I abort Web Form development way and move to MVC? When to move and when not?

4. What is the real use of URL mapping capability? What is wrong having extensions?

5. How to make use of MVC framework and continue using things like forms/windows authentication, URL authorization, output and data caching, session/profile state management, health monitoring, configuration system.

I am looking at the web cast from this site.