I ended up actually loving the XIII games as a whole. The first one was a disappointment in a lot of ways, but I still enjoyed it-- the ending however was a total bust. However, I thought XIII-2 came along, had better gameplay and structure, and made the story far more interesting. Lightning Returns was the best of the lot for me as it was a really atmospheric and different kind of game, but the way it resolved the overall story and characters was alright to me, so in many ways I thought it rectified the ending of the first game.
I personally look at the trilogy as one huge Final Fantasy game, and quite honestly one that I don't find to be bad at all.