It's a problematic pseudo-incest lesbian romance with abuse/assault elements. It's not trashy, though. It's not always very comfortable to watch, but I thought it was quite good.
Firstly, the pseudo-incest. Yuzu's mother has remarried and so she's now got a step-sister, Mei. Shout me down if you like, but that's not incest. That's two teenagers who've been shoved into the same bedroom. One of them's loud, gaudy and good-hearted (Yuzu) and the other's a self-loathing ice queen (Mei).
Next, the abuse/assault. You can either choose to see this as (a) messed-up people making a whole bunch of mistakes and hence legitimate territory for drama, or (b) disturbing fetish appeal for the author and/or the audience. It's not an offensive show-killer, admittedly. There's no rape or anything. Approximately, usually, it's people forcefully kissing each other in a sexy manner without consent. The victim afterwards might get angry, flustered or hot, but she won't be calling the police. That said, though, it's still a pretty alarming element and there's clearly something wrong with Mei.
The nasty one, though, was from the otherwise sympathetic Yuzu. She starts molesting Mei in a crowded train in ep.5 and I was cringing with NO STOP IT STOP IT DON'T DO THAT. In fairness, this is saying a lot about her character (and not in good ways). The show ends up being about how Mei and Yuzu have always been terrible at listening to and understanding each other. It's easy not to notice how similar they are as people because Mei's, frankly, always been unpleasant about it, while Yuzu's a bubble-headed ray of sunshine who keeps trying to help... but those attempts kept exploding in her face. In the first few episodes, this was comedy. Here, uh, it's less so.
(Mei's angry with her afterwards, but only in her usual icelike way. She then puts it from her mind and gets back to business.)
If you can take all that, though, the show's quite good. Its main problems are:
(a) Yuzu's the viewpoint character, but partly as a result we never know what Mei's thinking. Mei's a pretty opaque character and it would be easy to decide in early episodes that she's underwritten. (Even I found it implausible that Yuzu falls in love with her so quickly.)
(b) it can be a bit uncomfortable to watch. People make bad and/or hostile-looking decisions or, say, starting getting silly about indirect kisses when they've already had alarmingly direct, steamy ones.
(c) this is a small, subjective one that not everyone will agree with. However I found the plot capable of going a little bit too fast, cramming in developments in a way that could have seemed trashy. There are episodes that could have been improved by going more slowly and giving the characters more room to breathe.
For me, though, the show worked. The key factor is that I believed in Mei. She's like an evil, nitpicking headmistress. We're not seeing what's in her head, but that just forces us to keep our antennae alert for clues... and later episodes have some big ones. A shocking one, for me, was in ep.9, after she's just been manipulated by a bitchy love rival into doing some questionable things that could have made a nicer character look like an idiot. Mei, though, simply admits that on some level, she'd also been testing Yuzu. There's real darkness in her. Every bad thing she says about herself is true, but she's painfully aware of that and insecure under her chilly arrogance.
Similarly, the propositioning in ep.10 is shocking twice over. Its conclusion is enough to draw blood, of course, but look at how Mei went about it. It's a steamy scene, yet also ice cold. She's offering no human warmth. It's what she's not saying that says the most about her characterisation, as if she can't imagine that Yuzu might want to know what Mei wants or feels. Yow ow ouch.
At the same time, though, Yuzu's such a likeable fish out of water that the show itself doesn't feel cold at all. (She's a "gyaru", which in her case means flashy clothes, a ton of make-up, fake nails and a gift for infuriating stuffed shirts. She's about to transfer into Stuffed Shirt School. This can be hilarious.) I also liked the supporting cast. Momokino the Student Council Vice-President is shrill and often kind of one-note, but eventually lovable in how she overcomes her own hostility to become supportive. (She's also very funny. Great reaction shots.) She'd often be the best thing in an episode, or at least the most entertaining. Matsuri is flat-out evil... but then, underneath, she's not. The Tachibana twins are lovely, with Sara in particular being so nice that you'll dissolve.
I think it's a good show. You've got to stick at it, mind you. It's not a relaxing watch. It's walking a few tightropes with its content, although I personally wouldn't call it exploitation or trash. However I think it's telling a compelling story about fairly dark, complex characters. Its plot contains lots of recognisable romantic drama elements (the love rivals, the school trip, etc.) and is capable of rushing too fast, but for me the characters brought it alive.