I'd say it isn't a good game. Mechanics-wise, it's fine, and whether or not you find it entertaining is subjective. The problem, I believe, is the pacing and level-design. In the end, you have a marginally interesting combat system... and nothing else, really. The game barely lets you screw around with the sparse mechanics until half-way in, and even then the thing barely evolves. Given the ludicrous amount of time FFXIII expects the player to spend with it, the skeletal gameplay just can't hold up.

Plot-wise, XIII is a disaster, and since cutscenes are half the experience, the storytelling really needed to be a home run. I MAY be willing to overlook a little bit of gameplay bull if the story is engaging (Dragon Age) but XIII just doesn't get there. The story is arguably the biggest downfall of XIII, I'd say. The gameplay is adequate at best, but the slow realization that the story is a cliche-ridden, poorly-written mess just highlights that it isn't worth the player's time. There are far, far better game stories out there, and ones which even inhabit more solid gaming experiences.

Although I think FFXIII is a bad game, it's not bad in the way that, say, Superman for the N64 is - far from it, actually. FFXIII is exactly what the developers wanted it to be, so it really can't be criticized for being a failure. The developers should be commended for fleshing out their vision so well - it's just unfortunate that their vision ends up being so profoundly uninteresting.