Azumanga Daioh is really funny from time to time. As the other reviewers have said, nothing much really happens; it's just everyday Japanese high school. Everything is either leading up to a big humorous event, or IS the event. There's pretty much no emotion between the characters beyond simple friendship or enthusiastic, passing rivalry. It Is fun at first, but it gets tedious. It uses a lot of empty space (ex: "Did it cure your hiccups??" *long pause..."Hic!"*), which builds things up for a good laugh, but doesn't leave much room for other events. If you think you're okay with an anime that looks pretty good, has all the old cliches (sweat drops, jests about (...) size, a cat that appears now and again, the "impossibly good student", the cute girl, the quiet one), and is really hilarious once or twice an episode, then go ahead and get this one. Me personally, I'd prefer it if some sort of plot would develop.
It's been compared to Seinfeld...close, but not quite. Seinfeld has a lot of overlapping events and deals with an immense amount of situations, tying it all together at the end. Azumanga Daioh only covers that high school scene and goes in a straight line, with No Other Purpose but to make jokes. Sometimes, even humor gets old, when there's nothing else to back it up.
Put simply: there are (basically) no males, there is no main character, there is no point. A few good laughs, is that worth $27.00? I guess that's for you to decide.