I had the privilege of attending a two-day training for Expression Design and Expression Blend this week in San Jose, CA.  The trainer, Joshua, was knowledgeable and fun to learn from.  We enjoyed his training and personality.

Unfortunately, the product is just NOT ready for prime time.  I was shocked! Really shocked! Poor guy had to apologize almost every three minutes for two days on how the product works and why he has to do things in a very awkward way to get it to behave.
First the IDE is not intuitive at all.  I thought, hey I am a developer, maybe this is just not for me, but all the designers in the room were shaking their heads as well.

There was total confusion between canvas and layers in Blend, disturbing implementation for differentiating between selection and scope that could easily waste hours of work. There lacked in the IDE a clear path to redirect the effort to the correct path.
Look at the Trigger clickable button in the IDE in the picture below.

I thought you click that to access the Trigger page, nope, if you click on that Trigger button, you will delete the only trigger you have on your WPF form.  Who designed this IDE?
Yes I can see now the + sign and the minus sign but that is just not the way it is done guys.
At some point the IDE will be so cluttered with docked windows that it rendered the experience of working in the IDE totally useless.
You can save your designs as .Design files in Expression Design and export the XAML to Blend. That is a one way street; blend can not send the xaml back to design.  Expression Design does not know anything about .XAML files either and can not open them.  Just make them!
The scrollbar in the properties window disappears often and the user can no longer get to the properties off the screen space currently visible. Eventually, resizing the docked windows brings back the scrollbar.
Modifying XAML code in the editor sometimes does not reflect the change in the designer unless you close down the project and reopen.
Ok, that is enough, you get the picture.  Great idea, powerful product, long way to go to get it to be productive and useful.