Currently I am working on a project are using ASP.NET MVC, and I need reuse some user controls with Telerik Controls,
As you know, many of those user controls for WebForm need enable ViewSate.
So I had combined both new MVC and old WebForm pages in this project.
and I had encountered and solved many issues both for integration and MVC
I write some key points here as my collections and want to share with my visitors.
Here is an article about adding MVC to regular website :link

 1. setup Route in Global.asax

  
public static void RegisterRoutes(RouteCollection routes)
{

// routes.IgnoreRoute("Classic/{resource}.aspx?{*requestUrl}"); //the ? is not allow in the route config
routes.IgnoreRoute("{resource}.aspx/{*pathInfo}"); //All .aspx regard as webform.
routes.IgnoreRoute("{resource}.ash/{*pathInfo}"); //other type need escape
routes.IgnoreRoute("Classic/{resource}.aspx/{*pathInfo}"); //I put my webform files in classic folder

routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

routes.MapRoute(
"Default", // Route name
"{controller}/{action}/{id}", // URL with parameters
new { controller = "Home", action = "Index", id = "" } // Parameter defaults
);

}
2. Make your old UserControls work



3. Fix those weird problems of Visual Studio 2008
After copy the Classes such as "BaseUserControl.cs" to MVC project, whatever how many times I clean and rebuild the project, Those classes always can't be found by other classes.
And another issue happen frequently is the Pages have using Telerik controls, always have error message 'Error Creating Control' at design time.
After some research, I found  those step will solve these issues like a magic.

   step 1. Close your Visual studio 2008 .
   step 2. Go to C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\ folder, delete all sub directories.
   step 3. Go to C:\Documents and Settings\yourname\Local Settings\Application Data\Microsoft\VisualStudio\9.0\ProjectAssemblies\
              Delete all sub diretories.
   step 4. Restart.

And some people said, if copy the Telerik dlls to Global assembly will fix Telerik issue.