Structually, Azumanga is terrible, there's no real story line and quite a few main characters. However, it doesn't matter that there is no typical storyline, such as saving the world, as the reader becomes so attached to the characters, that they can't wait to see how they will do in their exams, no matter how trivial this may seem.
There are many characters, but each one has their own personality, and each is so fantastically funny in their own way.
It is difficult to describe the comedy of Azumanga, as it is unlike anything else. It also has a certain innocence to it that makes the whole series incredibly cute and heartwarming. This makes some people believe that it is for children, but it's not. There are a number of jokes in Azumanga that aren't suitable for children, but through these it still manages to keep its innocence. It's like when a cute kitten scratches something up.
To sum up, Azumanga Daioh is, in my opinion, the greatest manga ever, and will remain so for some time. Definitely worth your money.