The ending to XIII-2... Wow, that's a total "fuck you for playing the last two games" from Square Enix to fans.
Spoilers: Basically, it nullifies everything that happened in both games. I'm not even sure either game did occur in the game's universe. Either they have a something planned to fix things or they committed a massive story telling sin and left nothing for players to hang on to after all is said and done. It's beyond the world dying, the entire timeline is wiped out. There's just nothing to hang on to once the game is done, nothing to care about.
XIII-2 didn't surpass XIII as much, or at all, as it seemed initially. Initially, I thought the battle system was improved, but they actually neutered stagger, a big minus. Only a few enemies does stagger matter, most enemies that get staggered are almost dead anyways, those enemies you wish you could stagger often have a move to zero out the stagger bar, so can't be staggered. Brute force works better this time around, disappointing.
I was happy until the ending, though. Lots of improvements over XIII despite some steps back. However I don't see the point of an ending that makes you regret playing. If you want to cherish your memories of XIII, avoid this game, or at least wait until someone says they fixed what they did in the ending.