It's a schoolgirl lesbian comedy where nothing much happens. It's not the dreaded iyashikei, though, since it's also fairly manic and has lots of energy most of the time. (Apparently it goes a bit more iyashikei after Season 2, though.)
Anyway, I enjoyed it. It's quite funny.
Would I recommend it? Dunno. It's fluff. It doesn't matter in any way. The characters are fairly likeable, albeit sometimes loons. (Kyouko, we're looking at you.) They're schoolfriends who hang out together, do daft things and crack bad jokes. They're also all in junior high school, so the lesbianism won't be going anywhere for a while yet, despite the best efforts of certain filthy-minded goofballs. (Kyouko again. Also Chitose, Chizuru, and Chinatsu. This is not a show that's interested in closets.)
Apparently there's been a bit of fan politics about this show. This seems a bit crazy to me, but that's what I read. This concerns the extent of its lesbian content, which has apparently been called subtextual by fans who must be blind and deaf. Kyouko is a dirty old man in the body of a manic schoolgirl. Chitose gets a nosebleed every time she has salacious fantasies about Ayano and Kyouko, which seems to happen either (a) every five minutes or (b) uncontrollably until she puts her glasses back on. Chizuru is the same, but she drools. Chinatsu is... okay, you get the picture. Even the title is saying "lesbian" at you, since that's the manga meaning of the word "yuri". I suppose you could argue that the girls aren't actually shagging each other, but that would take us into more sensitive territory since they're 12-14 years old.
Well, apparently the problem is that it's got "yuri" in the title and was published in a magazine called Yuri Hime S, but it's also the magazine's most yuri-light series and yet is the only one to get an anime. That's more than enough to start internet wars in fandom, yes, but personally I don't really care. In practice, the real significance of all that lesbianism is to let anyone in the cast fancy anyone else. This show vapourises the Bechdel test. Its entire cast is female. Hell, its world is female. We don't see males and there's never any mention of them. One could argue that men don't exist in its universe and that human reproduction must involve alien pollen receptors.
Kyouko is the show's engine. Her brain isn't programmed to acknowledge the existence of self-control, or indeed of slowing down. She's a nutcase, but she's also brilliant at schoolwork (despite being almost incapable of the discipline required to do any) and she's quite an otaku. She draws doujinshi and fancies Chinatsu.
Akari is ostensibly the show's protagonist, but she worries about being insipid and inconspicuous. Yui is a boyish girl and the show's irritable sole voice of sanity. For her sins, she's Kyouko's childhood friend. Chinatsu fancies Yui and looks tiny and cute, but is very slightly yandere. Ayano is the student council vice-president and fancies Kyouko. (She's also a tsundere, but one of those rare tsundere who's perfectly nice about it and isn't annoying at all.) Chitose is the nosebleed queen who talks a bit like a granny. She fetishises Ayano, but seems to have channelled this entirely into dirty fantasies about her and Kyouko. You can get a lot of silliness out of that gang, but there are also two peripheral couples worth mentioning. Himawari and Sakurako fight non-stop, but you could cut the sexual tension with a knife. Then there's Nishigaki-sensei (a science teacher who believes that "explosions and destruction are essential!") and Matsumoto (student council president whose speech is inaudible and can only be understood by Nishigaki-sensei). These were some of my favourite people in the show, although unfortunately Himawari and Sakurako only get one episode to themselves and thereafter become background characters so generic that for a while I forgot who they were.
So that's the cast? What happens? What's the show's storyline? Ummm... okay, next question.
It's a successful show. It's had three seasons so far, plus a fourth mini-season with the Nachuyachumi! OVA. I enjoyed it. It made me laugh, e.g. Chitose's more extreme nosebleeds, which at their worst can look as if a small bird just exploded. There's potential twincest, but only if Chizuru gets her way. Above all, though, it manages to be aimlessly trivial without descending into "why am I watching this?" iyashikei pointlessness. The theme song is bouncy. The girls are nice. Admittedly Kyouko is a pain in the neck and almost impossible to get rid of, but she's nice too. It amused me. I'll be watching Season 2.