I haven't read the entire thread since I fear some posters haven't been taking care of posting spoiler-free impressions...
I've progressed nicely into the game so far. I'm about 5 hours in and my enjoyment has been increasing nicely since the beginning and as I'm getting used to the new battle system. I'm a bit critical though as this game seems to be very linear - more so than Final Fantasy X if that is indeed possible which is a huge disappointment.
The best thing about FFVII to FFIX was IMO the world map and being able to explore locations as you wanted in search of side quests. Sure, the story progressed on a linear path, but it was still a lot better than moving along on your very fixed path as XIII is doing it (so far).
The battle system is interesting. I'm still learning the ins and out but for now, I feel less central to the whole battle outcome than in other Final Fantasies. Even in FFXII, I felt more central to the entire battle by having to set up my Gambits (?) correctly to achieve an optimal result. In FFXIII so far, it's been mostly auto-battle so far and adjusting the overal strategic layout of occasionally bringing in medics, sabateurs roles into the gameplay. Selecting my own actions instead of auto-battle hasn't been necessary so far - and if I did, it's often too time consuming to be really effective. Auto-battle seems too effective not to use it, which IMO is a shame. This might change in later battles (boss battles maybe), but for now, I do feel a bit less involved in the battles.
I also haven't got much chance to level up either yet. Fighting the natural enemies that are spread across your path have been enough so far to gradually uplevel my characters. I'm not sure if this is enough, but half of the fun in older Final Fantasys were fighting randomized battled and leveling up your characters. Is this not necessary anymore? What a pitty.
Ah, the story.. well the story is interesting so far, but I definately haven't quite understood what, how, why yet. Or who for that matter. It feels a bit like FFXII which IMO isn't good, since IMO that had the worst and uninteresting story in any FF so far. To its defence, it's still early for me to judge and I have high hopes my impression will be changed.
Still all in all, good fun. Finally another FF and I'm enjoying it for what it is.