I just started playing with Expression Blend Beta 1
and you can color me very impressed! If you played with the older Expression Interactive Designer and came away less than impressed, much like I did, then the renamed Expression Blend is worth your taking a second look.
An attractive, cleaner interface combined with better performance leads to a much, much better experience. I've walked through the tutorials so far and have just found the experience to be much better. I am now getting ready to use it to build a prototype for a customer and think it will make my WPF experience fantastic.
I'm sure I will hit some rough spots as I go, but all in all, it think this could shape up to be the killer WPF design tool we have been waiting for. I'll keep you posted of my experience as I start to build the prototype.