I have kept red bellies in the past.

Beleive it or not, they are actually a boring fish. Not the eating terrors that you would picture a tank of these things would be like. I could put my hand in to clean the tank and move stuff around and never had a problem.
They will eat just about any meaty food offered but really like beefheart (you can get frozen cubes of it in a petstore).
I kept my group of 6 in a 125 gallon with a large pleco.
Some of the other species of pirahna are much more agressive and fin eaters and can't even be housed with their own types.
Check your local laws. A lot of states have now banned ownership of these fish (mine did or I would still have them).
All in all they are pretty nice looking and easy to care for fish.