Funny doesn't even begin to cover it. I laugh out loud every episode. This show just really cracks me up. It's just so absurd and lively and hilarious.
Story: There's no real continuous plot. It really is slice of life in that Seinfeldian "show about nothing" sort of way. Despite the fact that there's no real story line, it manages to find such absurdity and humor in what should be the simplest, most mundane human interactions and relationships.
Art: Even the art is funny. The characters' expressions and body language just adds to the unexpected humor of every situation, whether it's their over-the-top enthusiastic poses or the random adult glares they give each other once in a while. The art is also consistent and despite the fact that the background characters all look similar, I find that they're all easy to distinguish from each other and that they are consistent through the series, which I like.
Sound: Okay, if you've ever seen any of my other reviews or anything -- I normally do not pay any attention to openings and endings, except to generically note that I don't like them. I *LOVE* this show's opening and ending for some reason. It's bouncy and bubbly and absolutely perfect for the show. The opening is one of those cheerful running openings (like Gokusen) and it just hits the nail right on the head in terms of attitude and atmosphere of the show.
And the actors who do the voices of the three sisters do such a great job, especially the two younger sisters -- Chiaki is so deadpan and dry and Kana is so mindlessly enthusiastic. Perfect. =)
Character: I don't know if I've ever seen just a compilation of silly, hilarious characters in one show before. And it's really amazing that the show manages to balance all these ridiculous personalities together to work at making such a funny show without going too far and tiring out the audience. Even the supporting characters you see for one or two episodes, like the various classmates of the girls, are charmingly wacky and endearing. And don't forget the main characters! These girls are each so individual and carefully created that you can almost guess how they'd react in conversations they've never had in the show. They do a great job of subtly filling out all the details of their unique personalities that you get a very strong feeling that they're real and you know them, from the serene and nurturing Haruka to Kana's crazy, pedal to the metal perspective on life and the fact that little Chiaki makes you wonder if a dry, jaded old man somehow got stuck in an elementary school girl's body.
Enjoyment: I love this show. I've already rewatched episodes and it still cracks me up on repeat viewings. And I think other people love this show too, but this is just my guess. ;) Go watch this show!!