I tried MacRuby once a few months ago (https://github.com/tilltheis/fullscreenclock) and I have to tell you the Cocoa parts lacked some essential features (like correct syncing of Ruby and Cocoa setters). But the nice thing about it is that you can intermix Obj-C and Ruby code freely to write the low level parts of your app in C/Obj-C if you want to. There is even a bridge between C/Obj-C and Ruby which enables you to call native functions from your Ruby code.