There is a saying "never change a winning team", and that was exactly what they did. Everybody loved the charismatic characters and good stories, but that ended with the FF-XII (someone liked Vaan as a character?).
The production of XIII series is impressive, but the game itself is boring and the characters unconvincing. My impressions of FFXIII and XIII-2:
Still, I know I'll play every Final Fantasy game to be released, even if just to enjoy the production values