I like piranhas to look at, beautiful fish. But.... yeah I'd agree there over-rated. People who don't keep fish, if you ask them whats a dangerous fish everyones like piranha, piranha piranha....
They don't even seem that aggressive,except when their in larger groups... They always make me think of crack head muggers, you know the type who only every strike when their in much greater numbers then the enemy...
I like them in other people's tanks, but I'd never want any myself...
If you want aggression there's so many cooler more aggressive predators, if you want a good looking fish, again there's a million anyone would pick over a piranha...
I'm amazed so many people keep them!