I would say get it, but whatever you do, don't get it for the story. The BbS-esque battle system is crazy fun, and the amount of customization to do is almost frightening. The game is crammed full of post-game awesomeness including trophies and the totally-unecessary-yet-adorable Avatar system. You could probably play the game for a good 40-50 hours without getting very bored. I've already passed 20 hours, and I'm still working towards increasing my level, getting more keyblades, completing the stat matrix, and leveling up the keyblades I have. There's a fair amount of grinding involved, but the system sectors (new areas to fight in) are made more interesting because you have to bet points on your performance. The repeat worlds are a bit boring, but there are a couple cool new areas and the blox spread around shake things up a bit. Also, the new gameplay styles like the 3rd person shooter segment, the auto-scroll sidescroller part, and the turn-based RPG bit (all of Olympus Coliseum), are amazingly fun and succeed in keeping things fresh.
A lot of that is probably all gibberish to you, so I'll just say yes, it is worth it. If you've played BbS, you'll probably want to rent it to see if you want another go at a similar battle system (which does have some great twists, though), but if you haven't played BbS I would say it's a must.