WebForms work fine and if you like them, continue to use them.
Three of the big advantages to MVC model as I see it are:
ViewState is gone, which could create a fairly sizable amount of traffic over the wire.
URLs can be remapped to mean something as is all the rage now.
Scaffolding. I don't know, personally I think this is satan and encourages terrible programming habits, but other seem to think its a beautiful idea.
It also encourages a a proper separation between business logic and presentation by enforcing the Model-View-Controller pattern, but good WebForm code can mostly do that as well.
So, really, if you are fine with the overhead of WebForms, and ok with ugly URLs and don't want scaffolding, stick with WebForms.
EDIT: Oh, I did miss one major advantage of "clean" urls. And MVC application is much friendlier for SEO. It also gives you fine control over HTML, but frankly, I don't consider that much of a step forward.
