If you like cutscenes/stories and love Vanille, you'll love the game but if not, read for more info.
If you like cutscenes/stories and love Vanille, you'll love the game but if not you may have trouble liking the other aspects of the game.
Being able to enjoy the story/characters or not may make or break your enjoyment of the game.
You'll probably want to do your best to just "enjoy it" as much as you can because the game has a lot of cutscenes and dialogue. I felt FFXIII featured one of the best stories and it also featured my favorite character Vanille. However since story tends to be the most subjective part of RPGs (with stories), then that means opinions will vary a lot.
This review will mostly focus on the negatives of the game and if you can find that the negatives do not bother you, you'll probably enjoy the game.
Though one thing first "Game Length" - The main story will last for 35-50+ hours (depends on how much you level up or whether you just want to go through everything with minimal leveling). To get the "Platinum" trophy (or for Xbox 360, to get all achievements) it will probably take at least 80-100+ hours (though take note most hours are just from grinding/farming).
The rest of the review:
[1] Characters - I heard a lot seem to dislike Vanille, well I love her :). If you too love Vanille, you'll probably enjoy the game/story.
[In case you were wondering - Vanille has an Australian accent (her VA is Georgia van Cuylenburg who is Australian).]
In terms of characters in general though, you'll find that the characters display a wide arrange of emotions more than others in the series. This may be good or bad depending on your preference.
[2] Story - Like I said earlier, the story really depends on your tastes but I found it great if you could understand it.
There are some confusing things though (they're not explained very well in the game at times). So to get the most out of the story, you may want to read the wikia or other sources where all parts of the story are explained thoroughly. Datalogs (in game encyclopedia basically) do provide information you can read in game.
This isn't really a negative (to me) but I do know it may be to others.
[3] Exploration/Adventuring - There are no towns and there's no NPCs to interact with. The game is linear (it's basically a tunnel until Chapter 11) but the game does provide plenty of cutscenes and story.
Another important thing to take note of in FFXIII is that until Chapter 10, you can only control certain characters. You won't get to chose your party members/characters until Chapter 10.
You may find you'll become a bit lost once you reach Chapter 11. The difficulty spikes and regular battles can become a chore once you reach Chapter 11, then the difficulty spikes again in Chapter 13 (battles can take 3-4 minutes if you're not "leveled" enough).
Sprint Shoes are a must once you reach Chapter 11 [really look it up once you reach the chapter and get those ASAP :)].
I felt FFXIII is actually the most grindy game in the series (This excludes FFXI of course :) )
To get a maxed out weapon for everyone, to max out accessories, and so on you may have to spend 10-20 hours.
[4] Leveling and Customization - One of the biggest flaws in the game IMO is the lack of customization. Another one is that leveling is limited based on your story progression (so that means no overleveling). I know some people enjoy overleveling a bit so they can get on the story more easily (make random encounters much easier, etc) but you can't really do that much in FFXIII.
In terms of Customization, games to compare to are FFVII and FFX. FFVII had a lot of customization options thanks to Materia while FFX had a lot thanks to being able to customize equipment stats (as well as later being able to customize the sphere grid).
In game you're limited to 1-4 accessories (more unlocked later in the game through the Crystarium system) but that's very limited. You'll find you'll use two of those slots for Sprint Shoes (almost a must) and maybe Genji Gloves and/or farming accessories.
Essentially you don't have much room to customize as in FFVII or FFX for example.
[5] Character Balance - Unlike FFXII where the characters (even the limit breaks[Mist Quickenings in FF12]) were the same, in FFXIII all characters are different. The problem with this is that not all character are balanced.
Vanille and Fang (for example) are the only characters who can learn Protectra and other tier 2 buffs (which are really helpful in the optional harder fights in the game). While it was fine for me since I love Vanille, some may be disappointed that their favorite characters aren't as useful in certain cases.
Though know that you can probably defeat every single enemy in the game with almost any setup you like. So if you want (after Chapter 10 when you can chose characters) to play with Lightning, Snow, and Fang (for example) you can in (in some cases it might be really difficult though).
[6] Battle System - Final Fantasy XIII features one of the best battle systems in the series but you may feel that the game doesn't really take advantage of it.
In game you can take use the "Paradigm Shift" system which allows you to change classes/roles during battle (similar to FFX-2 but there are only 6 roles in XIII).
One problem is that for the majority of the game (before the end) you'll find that you won't be doing too much role changing (maybe to heal, then to damage deal, then sometimes to debuff). You may feel the system feels a bit "forced" due to this.
However there are moments (most noticeably during certain optional bosses) where you'll get to really experience the gameplay. You'll be switching and taking advantage of all six classes in battle (switching to buff, then debuff, then quickly back to heal and DD then to tank just before the enemy uses a strong attack, then back to healing and buffing, etc). In situations like that, you'll basically experience one of the best gameplay experiences (in terms of battle system) in the FF series IMO.
[7] Other Stuff - Eidolons are the summons in the game (and the "limit break" essentially). Each character gets their own summon.
The problem is unfortunately summoning is limited and based on TP. For the most part (outside of farming Adamantoise, healing yourself, and certain boss fights where you can poison the boss) summons won't be used often.
You'll find you want to Libra every enemy to get them in your datalog or simply save TP in case of emergencies for Renew rather than using summons. Even in (most) boss fights, you'll find that just attacking normally is faster and more effective than using a summon.
Another negative is all but Vanille's Eidolon has Curaga. Why Vanille's Eidolon does not have Curaga, I do not know but I find that to be a bit disappointing since she is my favorite.
So outside of farming Adamantoise, the summons generally do not get used as often as other FF games which I found to be a bit disappointing.
[8] Conclusion - I listed most of the negatives I've had with the game but if you don't mind any of those (and love Vanille), you'll probably enjoy the game.
The game's story is easily one of my favorites but I do agree things can be confusing at times especially if you do not read up on background or extra stuff relating to the story through datalogs or so. The music is great, Vanille is awesome, and the battle system can be really exciting.
Overall the game does have some flaws (which could have been improved on such as Eidolons for example) but if you can deal with the flaws, the game is definitely worth it especially if you're a fan of Final Fantasy.