Visual Studio 2010 is a huge release in terms of designer support for Silverlight and WPF. You can now drag Silverlight controls from the Toolbox to the design surface. You can resize and position controls on the design surface, set their properties and create event handlers using the Properties window and more. I've used Visual Web Developer and the Visual Studio 2010 Ultimate, both with excellent results.
In addition to core scenarios, you can do nifty things like change a control template and see the control its applied to update in the designer immediately. Also, when you install the Silverlight Toolkit, you get the same designer support offered for controls that ship with the runtime and SDK.
For an overview of the Silverlight Designer and links to other topics that cover some of the designer scenarios, check out Silverlight Designer for Visual Studio on MSDN.