First, my RPG history:
Final Fantasies I, III, IV, VI, VII, IX, X, X-2, XI (PS2), XII
Chrono Trigger & Cross
Super Mario RPG, Paper Mario, Paper Mario: TYD, both Mario & Luigi games
Xenosaga Pts 1, 2, and 3
Kingdom Hearts 1, CoM, and 2
Golden Sun & GS:TLA
Tales of Symphonia
Several .hack games
Chrono Trigger is absolutely my favorite RPG (and game) ever, FF IV is my favorite FF, although I really enjoyed most of the games I listed tremendously.
That's not a complete list, but at least gives a good overview of some RPGs I've played. Xenosaga 2 is easily my least favorite of those listed. That is perhaps the only game on the list I would place behind FF XII in enjoyability.
FFXII did several new things for a Final Fantasy. Its battle system was interesting I suppose, but handled improperly. I strongly believe that the battle system would have been enjoyable if you didn't effectively have idiotic AI for the vast majority of the game (until you finally earned a sufficient number of gambits). By artificially limiting the intelligence of everyone who wasn't being actively player controlled, I feel like the game creators took away a sense of involvement in order to provide a sense of action which did not adequately make up for something every previous Final Fantasy had: control of the entire party.
Don't get me wrong. I don't mind games where you control a primary character and only tweak some controls for others. Kingdom Hearts and the first .hack games (the ones I played) did this remarkably well. In those games, you control the main character and set some options for what you generally want your party members to do (prioritizing healing over attacking, items over magic, etc.). Those games, however, gave you as much control over the AI in the beginning as you'd get in the whole game, rather than FFXII's approach of making you micromanage to take advantage of any gambits you may have picked up that would help you out.
So the battle system was one negative. Next, is the story and characters. They were dreadful. The story was really bad for a modern RPG. There was no mystery to it whatsoever. You could figure out from very early on exactly what the outcome was going to be and who was going to be doing the fighting. There was absolutely no character development. None. I'm not joking. There was far more character development in Final Fantasy IV, a game that came out on the SNES. There was also more character development in Final Fantasy VI, VII, IX, X, and X-2 (yes, even X-2). There was one really compelling character (Balthier), one who seemed to feel irrationally guilty over something he had absolutely no hand in (and actually suffered because of), and another character who could have been compelling if her story was a bit more fleshed out (Fran). The "main characters" were terrible. Ashe is a whiny princess who doesn't grow at any point in the plot, until she miraculously changes at the drop of a hat in the finishing scenes. Vaan actually played no role in the plot other than being there. Seriously. He could have been taken out of the plot entirely and there are maybe only a handful of scenes in the game that would have needed altering.
So story is another negative. Keep in mind that fans of Final Fantasy games normally rave about the story of the games and generally are fans of the "choose items from a menu" fighting system that the series has always had.
As others have said, the graphics are very good for the PS2. Exceptional, really. Unfortunately, the game still doesn't look as good as FFX. Some of this is preference and some of it is a nature of technical resources. Let me explain: First, FFX's color palette is generally more vibrant than XII's. XII suffers from what I call the "realistic is brown" effect. Things, people, and areas seem to have almost a constant brown filter cast over them. That is entirely preference. Other aspects of the graphics stem from the very nature of the game. Due to the real-time battle system, Square-Enix could not stick to pre-rendered backgrounds, which they used in FFs VII-X. With pre-rendered backgrounds, you can fit as much detail as you can conceivably fit in the display resolution, no matter the limitations of the CPU & GPU. Unfortunately, with pre-rendered backgrounds you also can not change the camera angle dynamically. Because everything is rendered beforehand, it is only rendered from a particular set of points of view. This way, you're allowing for amazingly beautiful environments and can spend much more of the GPU's power rendering polygons for nothing other than characters and movable items (chests, for instance). This works just fine for other FFs because battles take place on a separate battle screen. With XII, however, there's no battle screen (battles happen in the same areas you run through normally) and the angle needs to be dynamically changeable during battle. This means that every single thing you see on screen has to be rendered realtime. This means dividing the polygon rendering previously only being spent on characters (more or less) amongst characters, buildings, vehicles, and everything else in the environment. This often leads to far more barren environments than in FFX, and less detailed character models. It was actually revealed while FFXII was in development that the characters used far less polygons than in X. It's simply quality texture work that helped them to look as good as they managed.
So, graphics are a plus compared to most PS2 games, but didn't appeal to me as much as X.
The soundtrack was fine. Heck, it was actually very good. Sadly, it still doesn't measure up to the last FF soundtrack Uematsu had a hand in (FF X) or any of Uematsu's previous work.
Soundtrack is another conditional plus. (+ compared to most PS2 games, - compared to X and previous FFs)
The leveling system was great. Very few complaints here at all. I liked it just as much as I liked the Sphere grid in X.
So, in the end, I'd probably give it a 7/10 overall. That is not a bad score in my book. It's a decent RPG, but I prefer every other FF I've played. I do mark it down (not in the score, but just in how fondly I remember it) for doing so many things worse than previous FFs and for changing so much for the worse. (I didn't mention it, but I don't mind getting rid of random battles at all. I love Chrono Trigger, Cross, and Tales of Symphonia for adequately ridding themselves of that antiquated convention. That is perhaps the only change for the better in XII in my opinion.)
If you like older style JRPGs a lot, I doubt you'd enjoy XII. If you want something with a focus on the story, stay away from it.
Other than that, I can't really give any other details, but I've probably posted more than you're willing to read anyway.
Edit: Like others said. It's not a bad game, it's just not good for a FF. I definitely recommend X as a great entrance to the series if you can find it (should be cheap). If you like X, then you should try some of the other entries as it really will give you a feel for the series.