If you ever want to try it, we're able to get most custom sites into sitebuilder with minimal changes. Not all - but most. We can do flash headers, flash menus, nested/drill down menus, horizontal menus where the submenus fly out below them, different images for each menu item, different header images (or flash header objects) on the top of each page, etc.
As a designer, you know the need for a consistent theme between the pages of your site (logo and menu location for example). Most of the other items we have been able to build into custom sitebuilder templates. So if you have any clients that want a custom design that looks more like a custom site than a template site - but they still want to make changes themselves - please keep us in mind. We have been able to make sitebuilder a very flexible tool.