Beat it today. Actually felt kinda short, only about 22 hours. I think XIII took like 40-50?
The Ending:

Wow, lol. (If anybody wants me to elaborate on the ending because they don't intend to play it, I will, but sheesh.)
Other thoughts:
I thought the battle system would be what kept me playing the game, but I didn't enjoy the battle system as much as XIII. The third-wheel monster thing never really gelled with me at all, so it felt like you were stuck with the same two characters for the whole game, and for whatever reason they had a vastly limited set of abilities compared to XIII. So eventually the battles get incredibly monotonous.
On the other hand, I liked the time travel stuff, at least to begin with. It gets a little nonsensical by the end, especially as they try to tell you that changing the future fixes the past and you're like huh how that makes no sense. Eventually you just roll with it, it's a JRPG after all, they make up rules as they go along.
I also liked the big city Academia, and although it didnt have much in the way of things to do, it was nice to have a nice big city to run around using FFXIII's tech, since FFXIII itself never let you go anywhere other than multitudes of corridors. It felt nice and alive with all the NPCs running around and talking and minding their own business. In fact, the whole game I felt like the game was intentionally marking off each of FFXIII's criticisms like some big checklist. Too linear? Time travel. No towns? Giant city. No mini-games? Golden saucer.
All in all, it wasn't terrible. Noel was likable, Serah was... Serah. The last set of boss fights were all awesome, though that final "dungeon" (if you want to call it that) was annoying as hell. Fights got boring for the reasons above, but things were open and nice to look at and you got to explore a lot so I didn't mind so much. But jesus christ that ending. I'd heard in advance that it was a bit of a troll ending, but that's one hell of an understatement.
The ending was written by trolldad's crazy drunk uncle or something.