The advantage of this .NET interop is that you can easilly add AJAX feature to existing C# ASPX pages.
Postbacks automatically replaced with AJAX
In the core of the r.a.d.ajax framework is a patent-pending technology, which can make selected controls on the page perform AJAX callbacks instead of traditional postbacks. The AJAX-enabled controls are not merely rendered with callbacks, but preserve their interaction with the server and relations with the other controls on the page. In fact, ajaxified controls do not really know that they are working with callbacks instead of postbacks.
A key disadvantage of other AJAX frameworks is the fact that you need to turn you application inside out and make major modifications in order to make it work with AJAX. You typically have to place Callback Panels around areas that need to be updated, set triggers, and manually invoke AJAX requests. Off course all of this within Visual Studio 2005 WYSIWYG design editor.