Bear in mind that Sitefinity uses RadControls for ASP.NET AJAX, RadControls for Silverlight and OpenAccess ORM internally, therefore the pages you design with it will incorporate our controls after all.
Additionally, note that you can use our AJAX controls in Visual Studio and outside of Sitefinity as a trial version, or download the AJAX bundle separately and utilize it for your VS development. Apart from that, you may go your own route and implement your custom solution that builds end user forms dynamically.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed