I'll go even farther than naming FFXIII the worst FF title and say that I consider it one of the worst RPG's I've ever played. It tries to distract from that with a decent level of polish, but shiny or not, a turd is a turd.
The gameplay is some of the most shallow, mind numbingly stupid stuff I've ever seen. You will make a grand total of three or four distinct choices in battle, all of which are fairly obvious, and that's it. That is the most choice you have in the whole game. The leveling is on rails, the levels are basically corridors, and the battles are so mind numbingly stupid in their lack of complexity that aren't bored in five seconds then you either have a very high tolerance for playing games where you just do the same four things over and over again, or you're just slow enough not to realize that's all the game is.
We can add onto that the fact that the story makes no real sense. Even if you read the datalogs to get the all important back story that you won't get from just playing the game like a normal, sensible, human being the various twists that the story takes, and the choices characters make literally do not make sense. There is only one character that ever does anything that makes sense and that's Sazh. The rest of them are a tangled mess of them choosing to do things for the dumbest reasons imaginable, and then often pulling a complete 180 for even worse reasons. They're inconsistent, unbelievable, and possibly the only thing dumber than the people who designed the battle system. And it's not just limited to the main characters. The villains are often just as bad, enacting plans that make no sense to achieve goals that they spend half the game trying to prevent you from achieving.
And if that weren't bad enough, FFXIII is also one of the ugliest AAA titles I've ever seen. Now the characters are certainly passable, and the environments, though bland, aren't completely hideous most of the time. But the enemy designs, and the art style applied to them throughout the game is some of the ugliest, most unreadable, inconsistent piles of crap I've ever seen. I honestly would like to know if the person/people responsible were fired, because if not they should have been. They had absolutely no design sense whatsoever and seemed to spend most of their time adding details that worked against the flow of the creatures bodies, and often made them such a cluttered visual mess that you couldn't even be sure what they were right away.
Long story short, skip them. They are some of the worst games I've ever played. They try to cover it up under some flashy graphics, but no amount of shine is going to cover up just how bad they are. FFXIII is basically a game that languished completely directionless in development hell for years until the team was forced to make a demo about a year and a half before it released. They rushed something out, and then rushed the game itself out in a bit over a year. Until that point, they had no idea what they were doing or what the game would even be, and it shows. It was so haphazardly slapped together that it's pretty much unmistakable that they just slapped some disparate ideas together, refined nothing, and shoved it out the door.