FFXIII is not a terrible game, but it's definitely not great and it's not for everyone. I came into it with high hopes and I was disappointed by the story and structure and pleasantly surprised by combat, art, sound and graphics. I also do like the characters, at their best. I feel like they're not bad characters, they were just terribly written.
The game is structured like an action game, i.e. it's a series of tunnels filled with combat and cutscenes. For some odd reason the different locations in the game reminded me of Sonic zones, and while they were memorable for me, the narrative was not. I feel like there is a pretty solid core story in there, as far as console video game lore goes. The writing was awful and made me cringe more often than not. Square's decision to design the game in a linear action game style combined with a weak story were really the killers for me. Honorable mentions to Chapter 10's boring, repetitive gameplay and a villain that was only half good. Finally on the downsides, 40 hours lends itself to a well paced, deliberate RPG, but not to the action game structure FFXIII takes; it's really drawn out at points and could've been much better with half the run time.
As for the good, the combat system is excellent. I know there's people who complain about auto-battle, but I used it once in my whole playthrough, and that was just to see what it did. I acted like it didn't exist. In a sense, it reminded me of the Easy Automatic modes in games like DMC and Bayonetta. The combat plays out similar to many things in FFXIII, like an action game. Instead of pushing buttons, you select something in the menu, but the fluid pace of the combat and the simplicity reminded me of a slow, menu driven action game.
The graphics, locales, the world in general I really enjoyed, the settings were varied, colorful and interesting, and character design was solid and well executed (graphically). My personal character gripes are with Snow and Serah's disturbing size difference, which made the already badly written romance subplot scenes entirely laughable to me, and Sazh's oddly shaped fro. Why couldn't it have been a sphere? I enjoyed the music but long for the exhilarating, goosebumps-inducing 'video game-y' tunes of the old FF's, nothing approached Fight With Seymour or The Extreme.
If you really have been itching to try FFXIII, but the lukewarm reception has been putting you off, do yourself a favor and buy it, rent it, whatever, but just play it and find out for yourself. It's that kind of game. There are elements some people will love, some will hate. I avoided it for months because of some of the seething hatred I've seen for this game, but it's honestly not that bad. Buy it, try it or avoid it? Try it.
For reference, I've only played the big Nomura/Kitase FF's, VII, VIII, X and XIII. VI is next on the hopper. And I've yet to find a JRPG better than Panzer Dragoon Saga. Also, this isn't meant to be some kind of comprehensive review, it's me taking a brain shit on my keyboard and helping people who are unsure about playing this game.