I'll start off by saying that if you haven't played, or haven't enjoyed the previous FFXIII games, then this probably isn't for you. There are few new main characters, and so this story revolves around ones previously introduced in the series, so if you didn't like the characters or the storyline previously, I don't think this game will change that. Having said that, those who didn't enjoy the battle system in the previous two games will probably enjoy this one a lot better, as it feels more hands on - you are in direct control at all times rather than relying on the AI. If you haven't played the other FFXIII titles, whilst I'm sure that the gameplay will be enjoyable, the story will seem confusing. There are short flashbacks and references to previous titles designed to refresh the player, but I don't think these would be sufficient to cover it for new players. If, like me, you really enjoyed the last two games in the series, I'd seriously recommend getting this - particularly after the cliff hanger ending of XIII-2.
This game feels really quite different from the others in the series. Instead of mainly relying on an auto battle function, with AI controlled party members, you really are at the centre of all the battles, with Lightning 100% under your control. Gone are the fixed roles (paradigms), they have been replaced with the much more flexible garb system. All of the garbs you obtain are customisable with weapons, shields, accessories and abilities - and this is where most of the character customisation comes in to play. You can set up garbs pretty much how you like, most of them come with one or two pre-set abilities, and it is up to you to add in others to complete the garb. This gives you great flexibility as to how you want to play and take on enemies. The battle system will take time to get used to, but eventually the timing and the rhythm of battles will come much easier to you, and you will be able to control Lightning in a more fluid manner. It's definitely worth taking the time to get to grips with the guarding system - you'll need it!
A lot has been said about the timed aspect of this game. I was apprehensive about this, however it adds a real sense of threat and urgency,particularly through the early parts of the game. For instance, I completed one quest with only a few in game minutes (about 30 seconds real time) to spare - if I was any later the subject of my quest would have met an untimely end. Little things like that really add to the excitement. There is sufficient time to go exploring though, particularly on the later days when the main quests have been taken care of.
As you'd expect from a Final Fantasy game, the music and graphics are first class. The finishing CG scenes are really quite breath taking. The in game world also looks great - make sure to check out the night sky in the dead dunes, it's quite a sight! As for the soundtrack, I cannot praise it highly enough. There are little nods to previous final fantasy games throughout, for example winning a battle wearing Yuna's garb will cause the FFX victory fanfare, and in the Yusnaan arena the crowd occasionally sing the original final fantasy fanfare.
I was going to write that perhaps the game was a little on the short side, however having just checked my play time it clocked out just short of fifty hours - that's for one complete playthrough. Completing the game unlocks the New game+ mode, where you can replay with your obtained equipment on a harder difficulty, which gives you more customisation options (weapon upgrading) and better battle rewards. I personally would have liked the main quests to last longer, as they are easily completed by half way through the time limit. There are some challenging side quests and optional bosses to keep you occupied though.
Perhaps the only criticism I can find for this game is the lack of post story play - it's new game+ or nothing (although admittedly the story makes it obvious why there is no post story play). That and the fact the game doesn't pause if the controller dies, as I found out during the final battle!
So to summarise, if you're a fan of FFXIII there is no reason not to pick this up! Perhaps the biggest compliment I can give it is that I was sad to finish it as I was enjoying it so much! I'd also recommend Piggyback's strategy guide, as its well written and provides lots of useful information to help you through and master the battle system.