I've been using Expression Interactive Designer (formerly known as Sparkle) since it was publicly released. I have to say that even though this is a tool targeted at designers, it really can be used by developers, designers, information architects, interaction designers and hobbyists.
It's great for quickly prototyping a UI and if you have a couple of web services or objects to push some data to you can quickly see how things look. Add animation, event triggers and styles and you can get a great looking prototype very quickly.
Below is a diagram to show the process of using VS.NET and Expression Interactive Designer together to create a WPF application. There are a couple of entry points, EID and VS.NET and those depend on what you are doing. If you're at Mix06 I'll be happy to talk about it.
Tags: Mix06