I liked Objective-C anyway, so using Objective-J was no big problem, and the NS* and CG* frameworks allow for a very rich user interface with a consistent and fluent look-and-feel. If you have experience developing for Macintosh or iOS, you are nearly there.
Cappuccino is a major open-source endeavour which relies on a wonderful & select group of developers; it all started with 280North. Learning it requires a sizeable effort –if rewarding– and I am still learning. I thank that community for creating and continuing development of this marvelous tool: