Overpowered as fuck. Paper factory is a really good unique building and even after like a dozen nerfs Chu-ko-nu are still probably the best unique unit in the game, not to mention that upgrading them means they still keep their double attack but without the low damage. Plus the extra bonus power from great generals makes them even better, and isn't a boost that requires a really specific playstyle or only exists in early/late eras. My usual go-to when I decide to fire up Civ V.