You CAN use RadControls in the user controls that you create for your site. However, you will not be able to do it from the Visual Studio toolbox. You will have to include a @Register directive in the markup of your user control and refer to the assembly containing the RadControl that you want to use. These assemblies are in the /bin subfolder of your project.
Even if you do not create you own user controls wrapping the RadControls, but use them by dropping from the Sitefinity toolbox, you will still be able to edit their properties using the Sitefinity property editor. You will be able to access the DataSource and similar properties to control databinding. For anything that you cannot achieve through Sitefinity, you can still revert to wrapping the RadContol in a user control.
the Telerik team