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.)
While it's hard to say where this card fits into Constructed decks, there are plenty of ways that this could see play in Commander. There are so many ways to manipulate counters in that format, and when it 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. Sadly it doesn't fit into a deck like Vorel of the Hull-Clade, which is blue/green, and abuses counters like crazy. But it does fit into many others, especially Enchantment-heavy ones that involve White.
Conceptually, this card is awesome. Play-ability wise it seems 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 I feel its not incredibly Constructed playable. The fun factor here is definitely great enough, however, for this card to see some casual and Commander play.