Thank you for the feedback.
We do not currently support Expression Web as a development tool since it still has some limitations when dealing with custom controls. We are testing the controls in this new environment and trying to find out some workarounds for the major obstacles (toolbox registration, intellisense, property grid, design-time preview)
There are quite a few articles and forum/blog posts which describe most of the workarounds/limitations. Expression Web Program Manager's blog is a good read:http://blogs.msdn.com/andrewj/archive/2006/12/13/custom-asp-net-controls-and-expression-web.aspx
We will be very happy to hear feedback from the community:
Have you tried using any custom controls with Expression Web?
What are the major challenges you face with this new environment?