Ah Metalworker, you broken, broken little robot man. There are a few of these "Reveal X" cards out there, but this is the most powerful of them all because it adds so much mana for so little. Reveal 1 card, it's a Mox Sol Ring. 2 and it's a Mox Scorched Ruins. 3 or more and it's a Mox Why-The-Hell-Did-We-Print-This. And all that mana for the low, low price of showing your opponent what you're about to play. The only real problem with this card is that it's a creature, meaning that it's slower and more vulnerable than a regular artifact. Other than that, it's further proof that the Urza's saga should have had more oversight.
Pros: Adds a bunch of mana for almost no investment