What's needed now though is an update to the Visual Studio XAML designer so that it'll actually work with user controls. If I add a user control to a page now it'll tell me the control doesn't exist in the namespace, and sometimes it'll even refuse to render the page, claiming Invalid Markup, even though the control does exist in the namespace and the project compiles and runs fine regardless.

So at the moment, as soon as you use a usercontrol, the XAML designer becomes useless, which is a major pain.