Saki is a great manga. Definitely worth a read even if you know nothing about mahjong, or don't usually read 'sports' manga (which is essentially what this is.) The characters are all memorable and lovable, and the drama in the series becomes extremely absorbing. I'm trying hard not to spoil here but there were panels and scenes that sent chills down my spine.
The main drama is centered around Saki's relationship with her sister Teru. Saki lives with her father in a sleepy rural prefecture, while her sister is a famous mahjong player in Tokyo. The reasons they're separated are not made clear, and is only gradually hinted at over the course of the manga. Their relationship is clearly not good (in Tokyo, Teru coldly tells her teammates she doesn't have a sister.)
I really don't want to spoil, so I'll just say the way the way the reasons for their estrangement are gradually hinted in flashback had me as much on the edge-of-my-seat as anything I've ever read.
The main plot of the story is that the mahjong club at Saki's school (which she joins) is trying to qualify for the nationals, which take place in Tokyo. Saki wants to qualify so she can settle things with her sister, and also fulfill a promise with another girl.
Take big note of the Shoujo Ai category and GL Subtext tag by the way. The yuri is barely concealed (you can leave your goggles at home) and the 'romances' are honestly the best part of the series. They're what really gets you invested and I like every single pairing.
The weak part of Saki is the actual mahjong. There is quite a bit of time devoted to the actual games, and they're pretty silly. Some of the characters possess supernatural abilities, and score improbable hand after improbable hand. I'm not gonna lie, some of it can be pretty badass, but most of the time it's just silly.
However, there is one masterstroke when it comes to the actual mahjong games. Usually I find sports manga boring because you know who's going to win, which undermines the drama in the games. Of course the underdog protagonists will win, right? So when the manga tries to invest you in the struggle, it falls flat. Well, Saki sidesteps this by taking the time to individually develop every. single. character. This includes every opponent. You love every character, and they all have intense personal reasons that make them want to win. The matches maintain absolute intensity because you don't want any of the characters to lose, and they all have a lot on the line. This is good stuff.