The first four episodes are intolerable, but after that it improves up to "boring".
Mari Wakatake is an awkward girl starting a new school. Things get worse when one of her classmates, Hagino Senkouji, tries to strangle her (and after finishing the series I'm still not sure why). Mari's reactions to this are so implausible that I actually found ep.1 hard to process and had to go back and see if I'd missed a scene or something. Mari doesn't tell the teachers, or her roommates, or indeed anyone else at all. Instead, she waits until they're in the dining hall in front of everyone and decides to throw a massive snit, slapping Hagino and throwing water in her face.
Later, Hagino arranges things so that she and Mari are sharing a room... and Mari stays there. I don't know about you, but personally I wouldn't sleep in the same room as someone who'd just tried to murder me. I'd find myself a bench in the park, or disappear into an attic or something. Or I'd just quit the school.
This is bollocks and Mari's an idiot. Admittedly, she has extenuating circumstances. She's the amnesiac sole survivor of a disaster that killed everyone else on an entire island, including her parents. A thousand people died. She doesn't even recognise her own name. She's had it tough... but so will anyone trying to sit through her hostility in the first four episodes. She does that punchable "pointedly ignore someone while saying hnn" thing, which isn't helped by other girls doing that too. When Hagino saves her from an SF robot spaceship at the end of ep.4, Mari's reaction is "why didn't you rescue me sooner?"
After that, the show gets more likeable. The schoolgirls start behaving like human beings... which is ironic since one of them's an alien. The Arume are an all-female alien race of lesbians (so they say they're heading for extinction if they don't do something) and they're even less interesting than Mari. I assumed that they were stupid, bigoted and/or evil, given their discussion of that island's annihilation.
"When the drive went berserk, the entire landing force was annihilated, as well as one of the Blue's navigators."
"And at the destination point, 800 Forimes died from the effects of the Emil Force Drive. I take responsibility for all of this." ("Forime" is the Arume's word for "Earthling".)
"Obviously! Of course it's your responsibility! Who cares how many Forime you killed, you murderer? Two hundred of our compatriots died in that landing force!"
The only Arume with a conscience about this is doing the Stoic Unexpressive thing, while the others appear to be up to no good but aren't even interesting when they're villainous. (Probably because they're all boring and have no personalities except for the tediously stupid one who clings to her bizarre interpretation of reality even after having been shown a video of what really happened.) The good Arume manage to capture Tediously Stupid and end up letting her wander around their underwater spaceship until she makes trouble. D'oh.
This is still more interesting than what's happening at school, though. The show's only likeable character (Michiko Kouzuki) has been talked into writing a play for her class to perform. This is exasperating but not unwatchable when she's struggling with writer's block and everyone else is completely mishandling the situation, but I started fast-forwarding once we were into rehearsals, production, etc. (It's assumed that the writer should also be the director. Michiko's a shrinking violet. Forcing her to direct will expand her skill set and probably be good for as a person, but it's still unnecessary and insensitive.)
This is the kind of drab, understated story that will be called "low-key" and "sensitive" by its fans, but is actually stupid. In ep.9, Mari and Hagino decide not to take the train home from their shopping trip and instead give themselves a three-hour walk. No problem so far. They wanted the walk. 1. Mari, though, is soon lame and limping, but keeps quiet about it until Hagino notices for herself. 2. Next, they get caught in a torrential downpour. (Hadn't they checked the weather forecast? Alternatively, couldn't they have called the Arume ship computer to get this information?) 3. and they leave some of their shopping out in the rain when taking shelter. Specifically, all the paper they bought.
Then, in the final episode, there's a school play and an alien invasion. Ahem, priorities.
Incidentally, the Arume have conquered Earth in the original Blue Drop manga, which has more explicit sexual content and sometimes approaches creepy hentai. This anime is set a thousand years earlier than the manga's era.
In summary: avoid. Lots of SF background and story points are unclear or simply missing, while the character work is theoretically good but dragged down by a shortage of likeable or interesting characters. (There's even a stereotypical bloody-minded disciplinarian teacher who can piss off too.) Probably the worst show I've seen to the end recently.