In Australia these fish are an introduced pest. They have taking over the native waterways, muddying up the rivers and basically making it hard for the Australian freshwater sportsfish. They are causing so much trouble that it is illegal to return a carp to the water dead or alive. A carp fertilizer has now been developed called "Charlie carp" which puts the dead fish to good use. They were introduced originally to make Australia seem more European and they have also been introduced into our rivers by well meaning pet owners who could no longer care for their fish.
From what I understand the problem also exists in the United States and I would love to hear more about it.
However, after saying all that, carp are a popular angling target in Europe and they grow mighty big and put up a very strong fight. This great video is for all those people that like to catch carp.