*Review only goes up to chapter 49, the end of the Twilight of the Gods arc.
Story: World leaders play mahjong with each other to settle disputes, debts, gamble and to decide the next Pope. That's the basic, bare bones plot, but it gets more ridiculous as it advances. By the time you hit Ragnarok, you think you've seen it all but Twilight of the Gods rears it's head and it just goes off into the ocean. It's silly, it's absurd and it's delightfully fun if you can get past how mad crazy this all is. It's not particularly deep nor should you expect that.
Knowing mahjong helps loads but I know squat about it. The lack of knowledge doesn't make it any less fun as the characters and events more than make up for it.
Art: Manly art. Makes the moves more awesome, characters more cool (Except for that wussy guy who is strangely aware of the craziness around him) and more exciting overall. Smooth, soft lines and features are stuff for the weak. This is all about looking cool.
Characters: 7/8 world leaders who play mahjong show signs of pure ham. Characters would shout out their moves, make grand gestures while making said moves and hell, Papa Bush flexed and his shirt disintegrated! This promotes manliness of political leaders in the world of mahjong, where beings of pure badassery would stop at nothing to be able to play.
Hardly deep but who cares? They are there to be awesome, and that's what they do.
Overall: If mahjong isn't your thing, this might not be for you, no matter how fun or hammy it is. If you can tolerate it, give it a whirl. The plot isn't anything to write home about, nor the charcters but really, it's all about making it as fun as possible; as exciting and as ridiculous as it can be. And I for one, had a blast.