If an opponent casts a spell with X in its mana cost, the converted mana cost of that spell takes the value of X into account. If you reveal a card with X in its mana cost, X is 0. For example, if your opponent casts Blaze with X=1, Counterbalance will counter that spell if you reveal a card with converted mana cost 2, but it won't counter that spell if you reveal a Blaze of your own.

If an opponent casts half of a split card (for example, Hit), and you reveal a split card where half of it has the same converted mana cost (for example, Hit/Run or Stand/Deliver), Counterbalance will counter the spell.