It seems that Ruby language and OSX Cocoa API don’t mix well in every scenario. It is common knowledge that making these two work together is as easy as using RubyCocoa or MacRuby. These however are not universal tools; let me show you some use cases below.
The important thing to remember is that the solutions above (i.e. RubyCocoa and MacRuby) are not libraries.
RubyCocoa is an OSX framework applied into the ruby environment through interpreter patching. Such architecture gives greater flexibility: RubyCocoa does not provide a set of Cocoa bindings, it builds a bridge between these two environments (with lazy loading and other benefits).
MacRuby, also an OSX framework, is an implementation of ruby language that is tightly integrated with OSX. It features better performance, many architecture improvements (Ruby objects implemented on top of native Objective-C constructs) and better support of standard development flow (e.g. InterfaceBuilder compatibility).
These two frameworks prove useful in a scenario where an OSX desktop application is being built. Notably MacRuby provoked many positive reactions (e.g. 3 areas where MacRuby shines vs Objective-C) and some people believe that MacRuby will eventually replace Objective-C. Sumarizing, it provides significant gains in code verbosity while sacrificing relatively little performance.