The chaps who came up with Nostalgia, a WPF app that allows you to browse photos on Flickr explain how they used the new Expression tool, Blend, to create a stunning user experience with relative ease.
They written about their experience with Blend and talk about the various stages of design they went though on the project.
Check out the case study here: http://www.microsoft.com/products/expression/en/community/spotlight/Thirteen23.mspx
I love these guys. Absolutely beautiful.