As I've never seen one before, I thought it was pretty neat. Wikipedia made me stop and think about it when they defined it as being , "engineless but motored units that take their power from another locomotive."
I also found it interesting to read, "Slugs are used in situations where high tractive effort is more important than horsepower, such as switching operations in yards. At low speeds, a diesel-electric locomotive prime mover can produce more electric power than its traction motors can use. Extra power would simply cause the wheels to slip and/or overheat the traction motors. A slug effectively adds traction motors to the locomotive, increasing both the pulling and braking power. Slugs carry ballast to increase their weight and improve traction. Large blocks of concrete are frequently used for this purpose, substituting for the weight of the now-absent prime mover.