Like Chimeric Mass, it takes only a single mana for Myth Realized to become a creature with power and toughness equal to the amount of counters on it. Unlike the Mass, however, Myth Realized only gets better with each noncreature spell you play and leftover mana spent to give it more counters (the counters can be placed at instant speed, too.)
When Myth Realized becomes a creature, any +1/+1 counters placed on it remain on it as long as it remains on the field. (This is the same ruling that makes Steel Overseer and Inkmoth Nexus best friends.) While it's not an artifact like Chimeric Mass, which has its advantages, this card can be just as powerful and amass even more power more quickly in some cases.
While it wasn't the easiest card to build around in Standard, people did try. What about in EDH? It would seem that Myth Realized would be at home in decks that manipulate counters. Sadly, it doesn't fit into a deck like Vorel of the Hull-Clade, which is blue/green, and abuses counters like crazy. It doesn't really fit into decks that would take advantage of the counters.
However, Myth Realized is at home in decks like Shu Yun, the Silent Tempest, which plays an enormous amount of noncreature spells. Daxos the Returned, an Enchantment-happy deck, is also a great home for this Enchantment. Narset, Enlightened Master also can find a home for it on occasion.
Conceptually, this card is awesome. Play-ability wise it's okay, as anything that gets better as the game goes along is pretty awesome. To be fair, the earlier this card hits the board, the better it will be. You don't want to draw Myth Realized in the end game, which is why it's not that Constructed playable. The fun factor here is definitely great enough, however, for casual and EDH play.