Nothing short of brilliant. Arguably BioWare's best game, and definitely one of the best games ever.
PROS
Easily the best part of Jade Empire is the story. It seems small at first - just a normal martial arts story - but it rapidly becomes truly epic. The brilliance of the writing is that its many twists (and its biggest one, which rivals anything I've seen in any game or film, ever) are all completely clear in hindsight (and make complete sense as they happen), yet you don't see them coming. Hand in hand with the story are the characters and acting. The writing is top notch, and the actors are, to a man (and woman), perfect. It helps that the supporting characters' backstories are all told in detail. None of the characters are flat: by the end of the game, you know exactly what makes each of them tick, why they are the way they are. All of this combines to give Jade Empire the rarest and best of qualities a game can have: it's the kind of game that really sucks you in, so much so that you completely lose track of time and can end up playing for hours upon hours on end, completely disregarding real life (along with such mundane things as eating or sleeping). The game that can do this is rare (at least for me), but Jade Empire manages it beautifully.
At the same time, the world the writers at BioWare have created is incredibly fascinating and detailed. While not quite as detailed as the one they would later create with Mass Effect, it comes rather close. It's quite simply a unique fantasy world, which is all the more notable since it's so beautifully Eastern-inspired, yet the game is by a Western developer. Tied into the world is the history of the Jade Empire, which is thoroughly established, yet thankfully leaves more than enough gaps for future games to fill in.
Jade Empire offers RPGing at its best. At around 20-25 hours, the main storyline is satisfyingly long - yet not too long, as it never drags - and there are lots of side quests to do, yet none of them seem disconnected from the main storyline. Doing them doesn't ever feel like a chore. At the same time, there are real moral choices that you have to make, and decisions often have shades of grey. It's a refreshing, more realistic take than many RPGs have, when they merely offer a good/evil choice. Things aren't always that simple in the Jade Empire.
As for the combat, in my opinion it's the best combat in an RPG ever. No turn-based nonsense here, hidden (as in Knights of the Old Republic I and II), or overt (as in the Final Fantasy series). The combat in Jade Empire is more action oriented, and blends seemlessly with the rest of the gameplay: when you enter combat, there are no loading screens, no transitions. You just instantly enter a combat stance when you encounter an enemy and start fighting. It's perfect, as it makes the whole experience far more natural than it would be otherwise. While not as good as the combat in an action game like God of War or Heavenly Sword, it comes pretty close. Some critics have derided it for being too simplistic, but that's what's so great about it. It isn't difficult to get into, and doesn't require endless memorization of difficult combos; it's just flat-out fun. At the same time, those who want something deeper or more complicated have things to try like harmonic combos or switching styles mid-fight. The rest of us can just have fun button mashing, (but button mashing in a good way).
Finally, there's lots of humor in the game, as evidenced by the end credits. They're flat-out the best I've ever seen, rivalled only by the end credits of Portal. When you beat the game, be sure to sit through them. It's good for several laughs.
All of this combines to give Jade Empire a high replay value, higher than that of most RPGs. And lots of replay value is always a good thing.
CONS
There aren't many problems with Jade Empire. Really, the biggest problem has to do with some of the evil things in the game being, well, too evil, and too spiritually disturbing as a result. I'm talking about two things: Wild Flower's sidequests/storyline (which may be the most disturbing thing I've ever seen in any video game), and the path your character follows if you choose to be evil (which in turn may be the most evil I've ever seen one can be in a video game). And I'm not talking about a cool or interesting kind of evil. I'm talking about something that's just flat-out disturbing. It's difficult to talk about this in detail without getting into spoilers, so I won't, but suffice to say, neither of these aspects of the game are for the faint of heart. If you're the type of person who doesn't like stuff like this that ventures into the occult, don't play as an evil character, and don't do Wild Flower's storyline (or at the very least, don't do the evil path of it).
On the technical side, some of the minor character models aren't nearly as detailed as some of the main characters are, and look far more cartoonish as a result. From a gameplay perspective, the only nitpick I have is that the flying/shooting minigame could've been a lot better, and shouldn't have been required to play through to complete Kang's backstory. But these are minor complaints.
Finally, there's the third ending of the game, the statue ending. The game has three endings: the good ending, the bad ending, and the statue ending. Suffice to say, the statue ending is the worst ending I've ever seen in any game, ever, and probably will remain that way till the end of time. You'll know what I'm talking about if you see it, and I'll leave it at that.
CONCLUSION
Jade Empire is a fantastic game that everyone with an XBox, 360, or PC should play. While at first glance it may seem only to appeal to martial arts fanatics or RPG fans, it really has something for everyone, and has one of the deepest, richest stories I've ever come across in any game. It's arguably BioWare's best game (though I do slightly prefer Knights of the Old Republic, its lousy combat system notwithstanding), and one of the best games of all time.