After the MSDN event in Dallas this week an attendee stuck around to talk about Expression Web. He's a designer and had been working with XWeb for over 6 months, since it first was available as a beta. I asked him what he thought was the coolest feature of XWeb, and he replied that it had to be the way you can view the cascading of styles in the Style Management task pane. Because CSS can be complicated, and no other tool out there gives you this, it makes it so much easier to figure out why something isn't rendering the way it should.
If you haven't had a chance to check it out, go to www.microsoft.com/expression and see for yourself.