Great game, but not as good as I expected.
New characters, new abilities, new maps, and an Assist mode, all good.
The biggest improvement must be the AI, as enemies are now harder than they were at the same difficulty level.
Many system tweaks, and personally, I think the most annoying one is how Chase works now compared to how it was before, in Dissidia.
Ex Mode bar fills up a lot slower, both good and bad, good because in player vs player, people cannot rely on it as much, bad because it makes fighting more difficult enemies to grind equipment (in Dissidia, getting the Genji set and Lufenian set was very difficult as it was), more difficult than it already was (even without the Assist function).
The worst news to find out is that, the Assist only character, Aeris (of if you prefer, Aerith), can only be obtained if you have bought Duodecim Prologus, meaning that people who cannot purchase from the PSN store, cannot FULLY unlock/obtain all the characters, which is quite demotivating (to me at least).
In all honesty, the game is great, but not as good as I had hoped for, and in all fairness, the new characters and maps are not that good (or maybe I am just not used to them?).
I have studied Computer Games Programming at university before, and from this perspective, I can say that Square Enix has been quite lazy with this game; too few new features, most were just updates and improvements (which do not take a very long time to do, if you are an experienced games developer).
The game feels more like an update/patch rather than a new game.
If I had to rank the games in order from the Dissidia line, I would say the best is the Japanese version of Dissidia, then the English version of Dissidia (had some annoying changes/tweaks), and then Duodecim (just fewer new things than I expected), and even though with that said, Duodecim is still a great game, I would have still bought it knowing all that (maybe because I am a fan of the FF line?) and I would recommend getting it.