I have a couple Knight Goby's right now. Nowhere near as vicious as an Exodon, but still some mean little guys.
I bought them because I've never actually seen one in a store, and I don't like to pass up an opportunity. Besides, they are going to a friend of mine with a 30 gallon once it's finished cycling. For now they're in the 10 gallon where I keep my feeders..... well, I used to keep feeders there. These little guys killed 5 Rosy Reds in 3 days. Now it's just their tank.