As a card advantage machine that's uncounterable and hard to remove from the board, it's great in all assortment of creature decks, especially token-flavored ones
There will be times where you can use that extra combat step to great effect, but there will be more when you have neither the time nor the mana, nor the board state to actually make this matter Not enough nonbasic lands in the format Giving a creature firebereathing isnt particularly useful unless that creature was also pretty good, but this does replace itself, so that makes the floor on it pretty reasonable, especially for a one mana card
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However, Mondo Gecko might live best in decks that want to take full advantage of both of the cards effects
These are always a hit
Whenever you cast : Effects that make even tiny spells a force to be reckoned with