I agree on the red bellies. My first piranha were rbp, they are cool at feeding time but other than that they are scardy cats. If I walk near the tank they would flip out and all gather in the corner of the tank furthest from me.
I now have 2 black rhoms and 1 at a fish store that will stay there for the summer, 15" big guy. I also have 1 - 6" Gold Mac and will be bring home a 10" Gold SPilo next week. All of the pirahna that I have are very interactive now, they chase fingers and always watching me and following me when I walk around the tank. I personally think they are awsome fish as long as they are not redbellies. You just have to watch around and find one with a good personality, luckily I have found the best fish store ever, near me. They specilize in piranha and you can get just about any kind you want here as well as central and south american cichlids.
The cool thing about redbellies are how fast they grow, I am going to pick up a few babies for my 60 gallon tank that is empty so that I can watch them grow. I swear you can see them getting bigger everyday.