Pokemon Bank seems like a relatively cheap way to get your Pokemon across state lines, but Nintendo will be sweetening the pot in an effort to get users on-board. During the Nintendo Direct presentation this morning, the company announced that it will give away Celebi to anyone who tries the Bank before September 30, 2014.
Pokemon Bank launches on December 27, and will have a free trial for the first month, so presumably those who get in on it early will qualify for the free Celebi as well. If you want to continue having a cloud solution the whole year round, it will cost you $5 annually. Nintendo has committed to the Bank being operational for future Pokemon games, so whatever you've caught in X and Y will eventually transfer.