FFXIII's main issue was it was linear and the story was told entirely through cutscenes. Many of its shortcomings were the result of sudden changes in development and the target audience. It wasnt perfect, but it was still good.
It's sequel's sole purpose is to 'fix' it's tribulations in the name of fanservice and for the most part it works, so long as you can get over the unfocused story. The combat was improved, putting more emphasis on ability variety and restoring many of the series traditions. Some of the 'fixed' aspects however feel forced and is by far (IMO) the worst aspect of the game for what it stands for. For the current price, it's worthwhile, but the original is more coherent despite it's flaws.