Azumanga Daioh is often hailed as the mother of all Slice of Lifes, and for a good reason. Not only does it pioneer the idea of "cute girls doing cute things" and "daily life activities with little plot," it does these two things right.
The lack of a story is its story. Azumanga Daioh is a classic SOL manga, where most of its content is dependent upon showing the day-to-day activities of its characters, their interactions with one another, and their general progression through life (in this case school life).
While it does have a central focus driving it along (the characters' journeys from the beginning of high school to graduation), this focus is subtle, and is often felt as an aftereffect rather than directly demonstrated.
Additionally, there is no romance subplot to be had, which I would argue tends to be the downfall of many SOL manga. As such, the reader isn't hellbent on the result of the relationship, and instead can focus on enjoying the day-to-day aspect.
The art is not good. Period. It is an old manga, and the techniques, technology, and style clearly shows. Don't expect breathtaking scenery or beautiful wallpaper-worthy panels. Don't expect adorable girls with detailed hair and clothing, flying in the wind. That said, this is the one manga where a lack of good art...WORKS.
This is due to one primary factor: The characters are supposed to be ordinary. Oftentimes, manga artists tend to make the mistake of trying to pass off their main characters as ordinary and unremarkable, but end up drawing them as clear protagonists, with amazing details and shading. In my opinion, if your MC's can be distinguished from NPC's, they are NOT "normal." This is not the case for Azumanga Daioh. Any of the MC's can easily pass as an NPC, and none of them can truly be called "pretty." Sure, some people can argue Chiyo and Osaka looks cute, but it often is due to their personalities than their actual character designs, which looks downright basic.
Yet, in this particular case, it only serves to drive home the theme of the manga as a SOL. Take for example the final graduation scene. The reader can really feel the ordinariness and humanity of the student body. They are not protagonists who will all become doctors, lawyers, researchers, or ambassadors. Some of them really look like they will fail at life, while others have an uncertain road ahead of them. This, I would argue, is the reason why so many readers often cite this as one of the most emotional graduation scene: because they can relate.
Of course, the art is not the only thing that accentuates the characters. Their personalities are also very distinct and down to earth, making them appeal very much to an average reader. You will like some, you will hate some, you will not notice some, and you will find some annoying; almost as if you were really looking at your actual classroom.
It was enjoyable, and how could it not be with all of the above reasons?
However, did I read it in one sitting? No. The lack of any centralized plot (including romantic subplot) does act as a double-edged sword, as it can make the manga somewhat repetitive and mundane, especially if the reader is conditioned by manga with more exciting plot development.
Some of the annoying characters can also end up getting under your skin after a while.
Nonetheless, it was a pleasant read, and especially hardcore fans of SOL titles such as Non Non Biyori will certainly find it endlessly appealing, although I would be incredibly shocked if such a fan have not read Azumanga Daioh.
Overall: 10. Classic. Well worth the read. Masterfully crafted.