I believe there are a few distinct areas where CategoriesAreGood, and there are other areas where CategoriesAreBad. Under what circumstances are categories good?

– MikeTrent

• Good for placing private methods so that they are not in the public h-file
• Good for extending a class’ functionality; for instance, adding your own string methods to NSString instead of making them into functions or writing a new class. Another example: adding data compression to NSData.
• Absolutely required for AspectCocoa (Uh … why is that a good thing? – MikeTrent AspectOrientedProgramming is nifty. Even if you don’t want to use it, it’s nice to have it supported. Since categories cannot be overriden safely, AspectCocoa sounds like an example where CategoriesAreBad? – MikeTrent You want to talk to somebody who knows about that, I guess (: apologies.)
• Using categories also speeds up compile times for large class implementations. –zootbobbalu
• BillCheeseman uses them to split large controller classes into smaller files.
• (�more�)

See also ClassCategories for general discussion on categories and what you can use them for; after that, it’s up to you to decide if CategoriesAreGood or CategoriesAreBad for your situation.