The only downside I see is that it doesn't do anything on its own, which means that, in addition to a big target on its back, even in the perfect conditions opponents-wise, we might end up not casting it because our other cards aren't good enough in that spot
On top of this already intimidating list of mechanics, whenever Captain America attacks, other Heroes you control gain +X/+X until the end of the turn, where X is the value of Captain America's toughness
However, with the soon upcoming Magic 2014 rules change the Clone can remain on the battlefield as a copy of Garruk
Would they become 0/0, or does the combo simply not change lands into artifact creatures
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Personally, I think Mind Stone is better than its more expensive counterpart because you can play it way earlier