Categories are one of the options Groovy offers to let you add dynamic methods to classes at runtime, most often than not in a temporary way (unless of course you launch your app with a wrapping category as groovlets do), so no need to mess with MetaClasses (though EMC gives you more control).

Prior to 1.1-rc-3 nesting categories had some problems, and if you overrode a method withing the nested category then you were for a world of pain as it blew up with exceptions. But as I said, that was before, now its a different matter, you can have nested categories and at the same time override a method from the parent category inside the child category. Why is this feature useful? think testing.
Here is a simple example on nested categories
class Greeter { def greet(){ "Hello Groovy!" } } class CategoryOne { static greet( Greeter self ){ "Hello from One" } } class CategoryTwo { static greet( Greeter self ){ "Hello from Two" } } def greeter = new Greeter() assert "Hello Groovy!" == greeter.greet() use( CategoryOne ){ assert "Hello from One" == greeter.greet() use( CategoryTwo ){ assert "Hello from Two" == greeter.greet() } assert "Hello from One" == greeter.greet() } assert "Hello Groovy!" == greeter.greet()
Simple, yet powerful.

Keep on Groovying!