There are choices to be made, and rewards to be won, and it's a neat addition, if somewhat gimmicky
Obvious contenders are Snapcaster Mage and the like - creatures that accrue their value when entering the battlefield or in response to an opponent's attempt at a maneuver, and can then be used to counter a new threat later
While this card will be an auto-include in my Sakashima the Impostor Commander deck, I'm also excited to see it played in Zendikar Rising Limited, where it potentially ends up as a dual-class party member, a copy of a massive creature you already control, or even a land drop when you really need it
Between the counter distribution on ETB and the general benefits of being in green, we should have no trouble maintaining the largest creature
8 mana to grow any of your creatures to unfathomable size and give them annihilator
Green How do I recur creatures in Green