I've read you can keep them in groups when they're smaller and they will act like cory catfish, but once they get bigger they will become highly territorial
I don't really know if its true, so I only got one, he was happy and very active.
He died though when I was treating sick fish, he ate all the extra food I was feeding for my pleco, and I found him floating... So be careful with feeding, cause they will eat anything, and I mean anything, and they don't stop! I was feeding mine crane larvae from the yard, flake food, algae wafers, shrimp pellets... it was really fun, and he was like a shark when it came to eating. He'd smell it right off and start swimming really fast and hunt the food down.
At least he died doing what he loved!
Oh yeah, as said they eat smaller fish, so most tetras etc will be gone eventually - if you want only bigger fish, I say they're definitely worth it.