TL;DR: It's written in a way where there has to be a sequel (FFXIII-3).
The ending is probably one of the least satisfying endings i've ever seen, and the secret scene basically describes what Caius' intentions were for the entire plot. The amount of effort you have to put in to get the secret scene is pretty rediculous, you have to beat the game at least once, and then collect all 160 fragments. After that you get a pop up telling you to go to an area at a certain AF.
The paradox endings are "what ifs" and don't really explain anything about the plot, some of them are funny, some are sad, but almost none of them have anything to do with Lightning/Caius. You also can't change the ending. I watched the steamers do everything from time reversals, to answering Live Triggers differently, pressing Circle or Square or nothing during the ending scene none of it has any effect on what happens.
From what I could understand the dialogue is pretty well written, Yeul is a very interesting character, she also wrote a few of the analects in XIII, and might have been physically present to get the ball rolling on everything. XIII-2 makes the entire "XIII" series a lot more interesting, brush up on your FNC mythos because the Gods are pretty important, Buniberzei, Lindzei, and Etro. Knowing the mythology will spread a bit of light on the events that occur, or the events that will occur in XIII-3.
Don't expect to really get to see much of any of the main cast, mostly just Hope and to a lesser extent Sazh. Hope is a bit more bare-able this time around though. Lightning has about 15 minutes of screen time with 10 of that being in the opening scene. I believe you only really get to see Fang and Vanille during a paradox ending. One interesting thing I did see about XIII-2 is that you get to learn a bit about Lightning when she was in the Guardian Corps.
Like I said earlier in my post, the ending is done in such a way where there has to be a sequel. Square-Enix doesn't have the balls to leave the story where it is now. That and it's too much of a coincidence that they registered the domain for XIII-3. The ending is not something you would expect from a game that is supposed to tie up everything. Anyway, I hope that answered your question without spoiling anything.
Edit: Serah's titties look weird in her new outfit, I hope they release her old one as DLC or at least release the Bikini one stateside asap.
XI Wiki


