Saki is a series that falls imbetween two genres. There's too much ecchi for those who don't like it, and not enough of it for those who do. I think this sums up why a lot of people don't like Saki. But what are they missing out on?
The comedy for one, is brilliant. The constant banter and random situations bring mostly everything you expect from a slice-of-life/school-club comedy setting. The suspense in the mahjong battles is great, and the often overexaggarated moves and crazy abilities symbolizing plain moves, defeats and victories are both well made, completely out of place, yet strangely fitting! Calling it a Mahjong series with Dragonball powerups is not so very far from the truth, and though this may seem completely crazy, it works really well. Saki would not be anywhere near the same if it were not for the crazy stunts you see everybody doing.
But the factor I think makes this series REALLY shine, is the sheer amount of different characters. There are so many different people playing matches, so many different groups and schools, and every single one of them has theire own unique skill or ability. And by the end of just 25 episodes, you know every single one of them. And this, I think, is a feat in itself. There are so many series that throw a mountain of characters into the meat-grinder with unique abilities and tricks and all that jazz - but they have no soul, if I can permit myself to be so quaint. By the end of the series, they are forgotten. Saki on the other hand, manages to pull this off perfectly. There are no shallow characters. There are quite a few stereotypical characters yes, but across the board, the character cast is truly well made. And this is what I think lifting the series to the glory it deserves.
As for the Ecchi. Personally I don't mind it, but yes, it is out of place, and I can see why this puts off so many. If it didn't exist, I'm sure Saki would score well over 4 stars overall here, because there really wouldn't be anything left at all to rag at. But at the same time, I think something would be missing. A token amount of boing never hurt anyone after all, did it? ;)