I've been toying with the PyObjC bridge to develop Cocoa apps for a while, but it's only since the 0.9 release has been on the horizon that it's become a really functional tool. The interaction with the Foundation and AppKit frameworks is effectively complete which brings all of the power of Interface Builder to bear during app development. Coupled with the Python libraries that are available, this is an outstanding environment for rapid yet robust application development.

The documentation is sparse but that's improving. For example, I'd like to know how to interact with other ObjC frameworks.