My biggest challenge once getting past the basics of knowing the language, and learning the framework is how to create advanced user interfaces. There doesn't seem to be info much out there for for the GUI end of Cocoa apps beyond the standard widgets. For example, how to create apps with a UI that can't be created in Interface Builder, something like Boxee on the mac or ConvertBot on the iphone. Should I use Quartz or OpenGL or Core Animation or just a hierarchy of NSViews? I'd love to see some articles on the best practices for creating these kinds of user interfaces.