excellent integration with xcode build process. For example, you can use "Zero link" while debugging your unit test. This allows you to dynamically load new versions of your classes.
This is quite nice, especially after being spoiled by the JavaUnit integration in EclipseIde and IntellijIdea. It integrates very nicely with ExCode. An interesting difference is that instead of having setUp and tearDown, you use the native to NSObject methods of init and dealloc. Also, you run unit tests by building the project, not by having a special class that you run.
Caveat: You have to open up the details part of the build window via the splitter, at least in ExCode 1.5.
Note: With XCode 2.1, Apple started shipping standard TestingFrameworks for CeePlusPlus and ObjectiveCee applications.