We have been using Telerik UI on our websites (www.ssw.com.au, rules.ssw.com.au) for a while now and it's been working well. But one of our latest requirements was to render the items dynamically from a data file (possibly coming from a database in the future) and that was really painful to change since each level had its own template. At the end I had to move the templates into code instead declaratively (I found this example which helped me a lot: http://www.telerik.com/support/kb/aspnet-ajax/menu/details/mega-drop-down-menu-bind-to-xml-datasource-).
The second requirement is to make the mega menu responsive which by the look of it Telerik UI does not support. If that's the case we'll have to complete ditch the Telerik Menu and create our own version of it.
Do you have any input on how to make the menu responsive? It seems the Telerik UI hasn't been updated for a while as well, should we move to Kendo instead?