While a spell is on the stack, X counts as whatever value was chosen for it. Everywhere else, it's 0.
August 4, 2014 12:06 p.m.
August 4, 2014 12:09 p.m.
Just to clarify, the value of x does not affect the cmc of the card.
August 5, 2014 6:02 a.m.
It does when it's a spell on the stack.
202.3b When calculating the converted mana cost of an object with an X in its mana cost, X is treated as 0 while the object is not on the stack, and X is treated as the number chosen for it while the object is on the stack.
August 5, 2014 9:34 a.m.
Although X is treated as a number on the stack, the X still doesn't actually affect the cmc. The cmc of X cars are always, only the number of non-X mana you spend on it. For example, no matter how much mana you spend on Mistcutter Hydra it's cmc will always be 1. You can look this issue up.
August 5, 2014 9:48 a.m.
I just posted the relevant rule. Please read it carefully.