I loved XIII. I haven't played XIII-2 yet. And I can't wait to pick up XIII-3 too.
Lightning is my favorite character from the XIII "saga", so I can't wait to play a game that has her in it again, because sadly XIII-2 didn't. Some may find her boring and a "Cloud knock off", especially since Nomura designed her to be similar to Cloud, but I find her different than Cloud though there are similarities. However, this game has a chance to differentiate her personality a little more from Cloud's personality and give Claire (Lightning) a different vibe.
I heard that XIII-3 would be open-world ala Final Fantasy XII or something like that, so it'll be fun to explore, which was one of the criticisms of the first XIII (linearity).