I need to create a desktop CAD application which essentially should have a nice modern GUI. I am thinking of creating a WPF application so that I can have a rich user interface.
With WPF, a lot is possible. You'll find a wide variety of looks to various applications due to the fact that, unlike Windows Forms, WPF can be templated and styled much like HTML. Actual designers can easily bring a look and feel which is very difficult to accomplish in Windows Forms. Naturally, since it is so flexible, the look of highly styled applications will vary a great deal from application to application.
That said, there are some very good 3rd party controls. All the usual suspects have control libraries for WPF: Telerik, Infragistics, ComponentOne, Actipro, Devxpress just to name a few. Specifically, Actipro's Property Grid is very nice. There is also an open source one which I haven't evaluated, so can't speak to. WPF can also be "themed" by applying pre-compiled styles to controls. There are example themes found here: http://wpfthemes.codeplex.com/.
Finally, WPF's strengths are not fully realized until you learn how to separate the view which gets drawn and managed by WPF and the logical abstraction of the view, called the view model. Josh Smith has a great article about this pattern, known as Model-View-ViewModel, here: http://msdn.microsoft.com/en-us/magazine/dd419663.aspx.