Single player mode is decent at best. I played a little bit and it's not bad, but I think I've been spoiled by the awesomeness of multiplayer. I think you'll find single player enjoyable, but definitely not of 'must buy' caliber. As a single player game, FF:CC isn't worth $50, IMO. Give it a rent first and see how you like it.

Also, the designs are only similar to FFIX in the way that they are SD style and super cute. The overall look is different.