A fun and engaging game that is worth every hour that you spend on it.
There are a couple changes from and additions to the original Final Fantasy formula:
A major change is the new battle system. The new system is less time consuming but also smaller in scale than the traditional RPG system. Even though a few of the enemies are quite massive, they just aren't as overwhelming as they are in, for example, Final Fantasy X. One really bad part of the new battle system is that offensive magick is almost completely useless past a certain point in the game. It tends to be a complete waste of MP because it doesn't go over the maximum amount of damage (9999). This limits you characters to solely using their weapons for hand-to-hand and ranged combat. There are also "super-moves" called Quickenings that deal heavy damage to enemies but they drain a lot of your MP (depending on how many Quickenings you have, you might lose 1/3 to all of your MP) Quickenings are also not so useful after your characters become high enough levels. The series' staple (summonable creatures) is still here. They are Espers and, to be honest, they are pretty useless eventually (kind of like offensive magick and Quickenings). The battle system, though different, is still quite functional and enjoyable.
An addition to the Final Fantasy battle system is the Gambit system. Gambits are really helpful because they allow you to sit back and watch your characters do pretty much exactly what you want them to and you hardly ever have to input constant commands. Some of the more difficult battles require you to manually command your character to do something but you generally don't need to do much. Some people say that this detracts from the gameplay because you aren't really doing things half the time but I think the Gambit system only detracts from the tedium of the really long battles that you will encounter if you really look for them. If you can find a good set-up for your Gambits, you won't have much to worry about in terms of easier battles.
Another addition is the License Board. As you kill enemies you gain License Points (or LP). LP lets you purchase Licenses from the License Board. You need Licenses to wear ALL the equipment and wield almost ALL the weapons and to use ALL the magick. Licenses are also used to boost your characters' stats and to give them item boosting abilities among other things. There are also special parts of the License boards that allow you to unlock Quickenings and Espers.
One really enjoyable thing about Final Fantasy games is the fact that you can play them for easily 100+ hours and you won't have unlocked everything. There are the super weapons, the best armour, the Espers, all the tiles on the License Board, all the minigames/miniquests, extra bosses, the best accessories, monster Hunts, etc. An extensive list of unlockables exists and it would be extremely hard to list everything. I'll just say that it took me over 100 hours to get only a couple people a full License Board (of course I didn't focus solely on combat for all 100 hours). I hadn't unlocked even a quarter of the total things in those 100 hours. There is a lot of stuff to get in this game.
The music is top notch here as usual. The soundtrack is quite extensive and enjoyable but it isn't the best I've heard.
The graphics are also quite good and possibly the best I've seen in a PS2 game (especially for such a large game). There are bustling cities, vast deserts, dense jungles, and dead marshlands, just to name a few locations. I've only ever seen minimal slowdown recently after having this game (and my PS2) for quite a while. The CG cutscenes are stunning (not unusual for a Final Fantasy game) and well voice acted.
The story of Final Fantasy XII is okay. It's not the best ever but it's not bad. It's like this: A clueless kid gets swept up into a rebellion and meets more experienced and more knowledgeable people who help him along the way to victory over the evil oppressive empire. It is, like many people have said, a lot like Star Wars. This isn't a bad thing but it isn't a groundbreaking story.
Overall Final Fantasy XII is a thoroughly enjoyable game and is worth every penny and hour invested in it.