Some of the complaints have mentioned its linearity, does that mean it is a bad game - no of course not. FFX was linear also but that was still a popular FF.
One of the best aspects of this game is the battle system, which is the most polished it has ever been. There are not towns akin to previous FFs where you can stay at inns to recover your health and buy things, but there is still the occasional town. Your health recovers at the end of each battle and all the shops (which grow in number as you progress through the game) are registered at save points, which are around almost every corner. This system keeps the game very fast paced, just like the battles, all of which is necessary to feel more immersed in the plot and the characters situations.
One thing that impresses me is that each role has a purpose, unlike a lot of previous FFs whereby magic was really just a gimmick towards the end, because physical attacks were much stronger. Summons and every role are now useful. Attackers - obvious what they do. Blasters do less damage than attackers but push the break bar up on enemies by using black magic to allow the attackers to do more damage. Enhancers buff your team, Jammers debuff your enemies and put various negative status ailments on them, Defenders, support the team by being the punching bags of your enemies while the rest of your team can even heal up or attack uninterrupted and Healers, fairly obviously use white magic. These are the names of the Japanese roles, i think the western roles are Commando, Ravager, Synergist, Saboteur and Medic respectively. You can change these roles using a system called Optima Change or Paradigm shift in the Western version.
The characters are some of the best that have been created throughout the series, and the lead, Lightning, dubbed as the 'female Cloud' is the best lead since....well since Cloud. The storyline is also one of the best.
You may have heard that there is no traditional levelling up, which is true: There is something called the crystarium/crystallarium which is a series of crystal spheres that contain various attributes, abilities and accessory slots, that doesn't allow a lot in the way of deviation from its set path - much like the rest of the game, however there is a different crystarium for each role, and to start with you get 3 roles for each character and towards the end you unlock all the roles for each character, which you 'level up' by using cp (crystal points) attained from defeating enemies, just like exp.
Lastly you can't ignore the beautiful graphics, which really do look great and at times it's difficult to tell whether what you are watching is FMV or in-game graphics. However you only think that when the graphics are in fact in-game, but when it is FMV you will know, as it looks like Advent Children, just with more colourful scenery.
Overall, i really enjoyed this game, more so than IX, X and XII, but I can't bring myself to say its better than VII and VIII (the earlier games i didn't like, apart from VI).
Probably a 4.5/5. It's definitely worth buying, still has certain down sides, but the linearity doesn't detract from its enjoyability factor and when you play it you'll know what i'm on about. Oh and the boss battles are the most challenging of the series.