I've been using Actipro's WPF Studio for a few months now and I am throughly impressed. The WPF Studio comes with a first-rate sample application (which they call the Sample Browser), including full source code. The Sample Browser shows overviews, feature summaries, release history, demos, and quick-starts for each of the controls. The demos show samples of each control, while the quick starts focus on certain features available in each control. Also, when viewing a demo or quick-start, you jump directly to the folder that has the associated source code!
Each of the controls themselves are well designed and implemented. The Ribbon control is definitely the best in the business and it now implements all of the required and optional features from the Microsoft spec. Each of the controls has enhanced designer support in Visual Studio 2008, which is awesome and makes the controls that much easier to use. In addition to the major controls, there are other smaller controls availabe for use in the Shared library (e.g DateTimePicker, ColorPicker).
I'm definitely a sucker for eye-candy and usability, but the controls and sample application themselves are only part of the battle. Actipro's support is extremely responsive, knowledgable, and willing to help. The associated documentation is well written and organized. With your WPF Studio subscription, you get any new WPF controls for free! I already got their navigation controls (an Outlook style navigation bar and an XP explorer bar) for free with my previous purchase. I can't wait until they release their docking control (which they recently annouced).