Unlike its predecessors, the DomainDataSource control is a brilliant feature that you can use with RIA services to have code-free access to your data from the ASP.NET page - without sacrificing the structure of your application.
Previous data controls required data access code in the control, so you ended up with SQL in the presentation layer. Not a good idea. The DomainDataSource control doesn't do this, and is a vauable addition to your ASP.NET/RIA services toolbox.
This post explains what the control is, and shows how to use it.