See, Vic_Viper, this is what gets people in to trouble.
Discussions like this imply that there is no in between: a game is either a bonafied hit or an inarguable flop. In reality, unless you're CoD, Wii Fit or NSMB, there are always positives and negatives. Different lenses with which to analyze the data.
And the same is true here. FFXIII's sales aren't fantastic: they also aren't disastrous. They're great on an absolute scale (i.e. compared to all other games), and mediocre on a relative one (i.e. compared to other games in the franchise, compared to development costs, and so forth).
What does that add up to? I dunno. I guess I'd summarize it as: "okay." But no one likes that answer, because it's so much more satisfying to say it's a hit (if you love FF) or it's a flop (if you hate FF).