6 mana to play anything your opponent controls is... pretty damn awesome. Even just 3 mana to exile a creature an opponent controls is really good, especially in black and green. Let's say your opponent just played a Phyrexian Crusader and you're really worried about dying from Infect. With these girls, you don't even have to worry about the -1/-1 counters since they exile before damage is dealt. The bonus with the Phyrexian Crusader scenario is that you can later play the Crusader since you're running black! Basically, there's not much that can threaten these ladies in combat. Outside of combat, they're still pretty beefy and scary. Being black, they're protected from most removal too. However, they are really vulnerable if you don't have the mana to exile since combat tricks can buff their opponent into a lethal being.
Pros: Exile on a stick, convoluted way to steal creatures
Cons: A smidge expensive, vulnerable without mana