I used to have one. He was in a 55 gallon tank I had with an Oscar, Jack Dempsey and a good sized catfish (not sure of species, he was given to me).
He was AWESOME!!!! He got to about 14" before he died. They're not legal here, but I used to know a woman who sold them and piranha out of her store's basement. I was young and didn't care back then...
But nonetheless, they are incredible animals. I've watched a handful of shows about them too. It's too bad they're so invasive, they'd make great pets otherwise.