I'm really happy to see microsoft implementing this feature.
A couple years ago at work I created a tool to do this within a single (Caliburn Micro) WPF app. We called it the Dynamic View Reloader.
It basically intercepted calls to caliburn micro's ViewLocator and either fell back to the default implementation, or Xaml Parsed the (sanitized) xaml for the view. The results felt rather magical and was a huge productivity boost for visual/layout changes.
I've been hopeful since that microsoft would make that sort of feature a priority.
Apparently today is the day, bravo.