Final Fantasy XIII: Sell me on it

A guy was playing a demo of this game at Gamestop lastweek during a midnight release.
I was blown away by the graphics and music.
I'm thinking about buying this game.

What type of replay value does FF games typically have?
I've wasted too much money on games that were labeled "Must own" but had very very little replay value.

Help me out guys.
Why should I buy it?
Why shouldn't I buy it?