At long last, Expression Interactive Designer is available to the public at large.  My thoughts on this app:

1. Expression Interactive Designer is a showcase Avalon app. This is, hands down, the most complex WinFX WPF Avalon application that has been built to date.  It is entirely built on managed code, entirely built using Avalon itself.  If you are looking to demo what is possible in WPF, check this application out.  It represents the most sophisticated work on the platform.

2. Expression Interactive Designer makes Avalon animation come to life.   Avalon has this incredible animation engine, but without a tool, its power is locked up.  With EID, the animation engine comes to life and you can see the possibilities of the platform.  The timeline support is incredible.

If you are an Avalon afficianado, go download this today!

Little known fact #1: If you already have the Dec CTP installed, you can run the Jan CTP of EID.  So, you don't have to get a new version of WinFX. 

Little known fact #2: Every .NET developers hero, Lutz Roeder, whose .NET Reflector (which if you don't have it go get it now) is an essential development tool, is on the team that built EID!