Just my brief opinions (I could go on for ages and, indeed, once I did in a thread of my own creation a few months back):
Plot: Best of the FFs. The role the Occuria play make it feel very SMT (which is awesome) and the general tone of the whole thing just surpasses whatever came before it, especially when compared with the ham-handed delivery of VIII and Xs plots. Shame about the terrible pacing, at certain points, and the...
Characters: Ashe, Basch and Balthier are some of my favourite FF characters, and Gabranth is probably my favourite villain. Their development is subtle and believable - much better than the clumsy way character development had been dealt with in some previous entries (*cough* VII and VIII *cough*). It's, therefore, a great shame about the other 3 party members who get very little development and have no real reason for being there (except maybe Vaan, who plays quite the symbolic role). Half excellent - half poor.
The game was made by Matsuno, so it's no surprise about the quality of what is actually decent but it is striking how poorly some of it is handed (perhaps because he was taken off the project mid-way by Square). If you haven't already you should check out his other games, FF:Tactics, Vagrant Story (which I still need to beat) and Tactics Ogre. I just pre-ordered the remake of Tactics Ogre, and FF: Tactics probably has the best story of all of the Final Fantasy games. In my opinion, of course.
Gameplay: Decent but never blew me away. I enjoyed it, though, and think that XIII could have benefited from the Gambit system and having the battles in the same field (amongst the other things that dreadful game needed). As you can tell, though, my main "passion" for FF XII lies in the story elements.