Those are bad news!! They out compete a lot of the native species. There's worry that they're going to enter the great lakes which would be very bad. There is a canal to Lake Michigan that they reversed the flow of water a long time ago and there is pressure to revert the flow to its original ways to prevent the spread, but the powers that be (last I heard) are fighting to not do it.
On a side note, there are a lot of people that are bow fishing (shooting them w/ bow & arrow) as they cruise in their boats.