My old roomate had a pirahna, Nasty little fish, (he kept in in a 33 gal, was 4-5" across) Chewed down about 10 'medium' feeder goldfish a week... Made a huge mess of the tank, got stressed out by any light. (It finally resided under the light of only those silly walmart lasers)
He liked the darn thing, I thought it was ugly and a waste of tank space... But thats just me, I like lots of fish in my tanks.
I seem to recall in some areas special permits were required to own these fish... Might have to look into that.
Andy