Great game!
Anyway, this game overall is exceptional explaining a huge deal of hidden stuff about #13 Roxas and the unknown #14 Xion. Well there is one bad point I have, it's just depressing near the end heh, rly sad ending from day 350-358. But I won't include any MAJOR spoilers.
And also though i recommend this game, I don't think it was a NECESSARY installment in the series, but fun as heck.
When you beat the game and get to run around as cloaked, dual wielding Roxas, its just so badass.
Lastly I will leave you with that despite any flaws, this game has GREAT gameplay, dialogue, storyline and deep characters (despite them being Nobodies and not rly having a heart to even be DEEP) I recommend it!
Have Fun!