This is one of those series that makes you wonder just how much the creators were taking it seriously. The answer to that question can probably completely reverse your opinion, if you're like me.
The plot is quite basic, and for the synopsis I'll just point you to the main Anime-planet page. Around the main story of Shizuma and Nagisa, unravel the ones of a very large cast of diverse characters. It could be summarised as a convoluted dodecahedron of senpai-kohai/best friend-best friend relationships which more often than not gain (lesbian) romantic tones. A romantic slice of life, if you wish. It starts off as quite comedic and light, but as the plot progresses and backstories unveil it can get also quite sad, angsty, and also pretty moving. What bugs me is that, expecially in the first part, they play so many tropes so unbelievably straight (even with yuri characters) that many times you'll ask yourself whether they were doing this with a straight face (or rather, a lesbian fac... yeah, there's gonna be more of these horrible puns) or with the intent of doing a stealth parody. Because, if you accept they weren't taking themselves seriously, then you'll be able to enjoy this series as you'll be moved by the most well-done moments (be they sweet, funny or sad) but you'll also manage to have fun watching the most absurdly overdramatic and cheesy events; but if you don't... well, it'll seem just lame, if not downright stupid. But yet, if you don't take it too straight (however straight could a yuri get) the plot can get quite involving. If some gratuitous shots of Shizuma's gargantuan mammary apparatus wouldn't at times break the mood, it would be even better. It could have been more also if, for example, any single reference to the lesbian nature of these loves was made; there is no confusion, no hesitation, no self-questioning, not a single mention of the fact that the person they're taking a romantic interest in happens to be of their same gender.
The anime has got loads and loads of characters, and once again, they all incarnate so perfectly all the various stereotypes that you'll ask yourself if they had a bloody checklist or something. Again, though, they are such straight examples (well, not completely straight...) of such common tropes, that if you accept the postulate that it's not meant to take itself seriously you are sure to like and grow attached to at least SOME of them. Be it the cute innocent angelic girl Hikari, the masculine athletic Amane, the cute stalker with a crush/best friend Tamao, the evil lesbian duo Kaname/Momomi, the energetic and smart Chikaru, and so on and on. Being so standard, they can be easily identifiable and likeble, and thus their romances, and there's A LOT of shipping possibilities here, will keep you involved and interested... expecially if you're a boy, for obvious reasons (you know, this one's kinda explicit for a yuri... they actually kiss and such). It's a triumph of moe. A handful of characters will also go through some development and backstory exploration as the romances go on, adding some spice and depth you probably wouldn't have expected judging from the first part alone.
Even if the character design is clean and charming, the art and animation are nothing worth taking particular notice of. It's decent, with some low points like most series have, even if a bit stiff and sloppy at times in some movements. It gains points for some beautiful landscapes and rainstorms, though.
Throughout the series, the soundtrack is really solid and beautifully baroque-sounding, punctuating effectively many of the most intense moments. Actually, some famous classical pieces (like, for example, Beethoven's Piano Sonata N°14) are used. Both the first, extremely catchy opening (Shoujo meiro de tsukamaete, spoilers in video), and the second, neoclassical-sounding opening (Kuchibiru hakuchuumu) are very good and have that bittersweet feeling I find fitting with the story, while the endings (sung by two of the voice actresses) are terribly out of place in my opinion... because when an episode ends with a character crying after a very depressing flashback, a light hearted and happy moe ending really, REALLY breaks the mood.
The voice acting here is a bit generic, even if generally solid. There are a couple of exceptions though, in both ways: while Yuko KAIDA's Amane Ootori doesn't fully convince and Mai NAKAHARA's Nagisa Aoi can even be annoying at times in her lack of depth, Ai SHIMIZU's performance as Tamao Suzumi is convincing in her wide range of expressions (in some scenes, I was surprised by how much and how easily I could understand that the character's feelings were the opposite of what she was actually saying and trying to make believe!), Hitomi NABATAME's (Kanade in Candy Boy among many others) performance of Shizuma Hanazono becomes really deep later on, and particularly convincing is also Natsuko KUWATANI (Ryoko Asakura in "Suzumiya Haruhi no yuutsu", Suiseiseki in "Rozen Maiden") as Yaya Nanto.
Overall, I'd say this is a perfect anime for a first try at the world of yuri: being more light-hearted and less angsty and tragic than most yuri, full of (hopefully) not-so-seriously-taken clichés, more explicit than most yuri in regard of kisses and such, it's a nice first dive. Those who are already yuri fans, too, will probably enjoy it. It's not particularly deep or complex, but if taken with a bit of "genre savvyness" and humour it can be enjoyable and involving. I heard some people complaining about this show attacking the pairings that get "officialised", but while I too was annoyed by the main two couples (I was rooting for the other possibility of the triangles, so to speak), I think that if an anime manages to create different possible relationships and then make you upset (or happy) because you were (or weren't) rooting for another one... well, then there MUST be something they're doing right with the characters, isn't there?