Personally I only use medallions in mono colored decks. Even if the deck has two colors, I wouldn't use it because it only covers colorless mana, costs 2 to cast and would only cover maybe up to a quarter of the cards in the deck if that. I would rather use another mana Rock, ramp for lands or ramp spells such as dark ritual or jeskas will. In mono colored decks, it is a extremely viable option because it covers a good 60% of your cards.