Expression * products are meant to produce [primarily] raw XAML files that can be later consumed into ASP.NET applications as modules. They are also supposed to expand when Silverlight 2.0 comes out [animations and the like] and both VS and E* will be complementary. I went to one of usability studies for MS, and basically the Expression * products along with Silverlight are meant to tackle Dreamweaver and Adobe Flash collectively.
At a higher level, it's just an extra tool at your disposal. The only major difference I have seen is the ease of producing raw XAML in Expression Studio as opposed to doing it manually via forms designer in VS2008. I guess it also depends on your knowledge of CSS and HTML. I usually tend to wade through the markup manually, and use the designers minimally (main reason being they don't render the end result accurately, especially when dealing with IE vs. FF issues) - so I can't comment on the differences in the designers.
Hope this helps.