This is surprising to me because I'm rather ambivalent about the game as a whole. There are some aspects of the gameplay that I feel are well-designed; in particular, battles do a great job at maintaining tension, and I really do like the role-based nature of combat even if I have mixed feelings about how it plays out in practice.
On the other hand, we have parts that were just terribly designed: 90% of the items in the game are a complete waste (almost all upgrade materials, the vast majority of accessories, and too many of the weapons). Your paradigm deck is the only aspect of battle preparation that makes a difference in most battles. Equipment is almost always mindless: you'll do just fine equipping your best stat-increasing accessories in all but about three battles, all of them optional. In other words, a no-equipment run of this game plays out exactly the same as a standard run, which is just incredibly poor design.
That's my largest gameplay complaint. If you can enjoy the battle system despite its flaws, then FF13 is a really good game. The cast is the best the series has had since 6, which basically means it's in the top three. (Where it fits into that lineup is going to be a matter of preference; for the record, the other game would be FF4.) The plot itself is convoluted even by the standards of the series and/or genre, but it's one of the few games I've ever played with real, honest-to-goodness character development -- and I don't mean just backstories and stuff, but actual growth and progression over the course of a narrative -- so if you're willing to evaluate the story on those terms, then it's easily the best in the series.
Much has been made of the game's difficulty. It's true: the main game is the most difficult in the series, offset heavily by the low penalty for failure. The optional content, however, is lacking in difficulty; as mentioned above, the game offers you power that far exceeds what is necessary for the completion of these challenges. Wisely, though, the game makes most of this accessible only after you complete the game, meaning that you won't completely outlevel the final boss.
So, yeah, the game has flaws. But there's nothing here that's major enough to make the game unenjoyable. It doesn't scratch the same itch as many of the other games in the series, but that's exactly why the series is so enduring: it tries new things.