It's a magical girl show. I loved it.
Firstly, the franchise. Fate/stay night is a series of adult visual novels (i.e. computer games) with lots of spin-off anime. The latter aren't pornographic, by the way, but they are quite dark, adult and violent. I watched an episode of it last year and got bored. One day I'll try again. However this particular franchise branch is a witty, light-hearted spin-off that reinvents some of the characters as magical girls.
Less surprisingly than you'd think, men are the target audience. Magical girl shows had always traditionally been aimed at young girls, but these days people like me have become a significant peripheral demographic. Sometimes it's hard to guess the target audience. Almost any magical girl show is liable to have fans from both demographic groups (and often others too)... but as it happens I'm pretty sure this particular show won't ever be on children's TV. It's not sleazy or anything, but there are some off-colour jokes and the later seasons will have fanservice. It's ignoring the traditional magical girl demographic, which I think is a shame since the show's excellent and I'm convinced little girls would go apeshit for what I've seen so far. It's a fantastic show. Dramatic, engaging and funny. Season 1 is also the most family-friendly, with fewer lesbian schoolgirls and no panty shots, although it does have some mild sex comedy that's laugh-out-loud funny but unlikely to please the majority of parents.
The show's also an affectionate semi-parody, both of the magical girl genre and of Fate/stay night. It starts out entertaining and funny. Later it gets darker and more emotional, but it ends on a happy, feelgood note and never strays too far away from laughs.
Ilya (full name: Illyasviel von Einzbern) is a half-German girl at elementary school. Ten? Twelve? She, her family and even her maids have albino colouring and red eyes, but I don't think they're a nest of vampires. One day a flying doorknocker called Ruby talks her into becoming a magical girl, while also making incest jokes about Ilya's older brother. (Ruby's persuasion techniques would get you arrested by the trading standards authority if you tried to use them in a workplace.)
In fact Ruby already has a perfectly good human assistant, Rin Tohsaka, who's also far older and more experienced than Ilya... but unfortunately Ruby has a sibling called Sapphire and their two humans don't get on at all. The other one's called Luviagelita Edelfelt and she couldn't stop squabbling with Rin if you glued her mouth shut. Forced to choose between experience and common sense, Ruby and Sapphire decided that scamming ten-year-olds was a better bet than relying on this pair of retards.
All this is great. Ilya watches magical girl anime on TV and gets a kick out of being able to cast spells and fly, but also finds it embarrassing. If you had a nude transformation sequence, you'd run into the toilet too. Meanwhile Sapphire's victim is Miyu (the only regular character with no direct Fate/stay night counterpart), a superintelligent and apparently emotionless girl whose only weak points are:
(a) no imagination, which is a handicap when trying to do theoretically impossible things
(b) truly horrible communication skills
(c) self-sacrificial tendencies, taking everything upon herself and playing the martyr
(d) being a dork
I love the show's tone. It's playing with its own genre awareness and full of character-based jokes, so for instance Luvia's idea of teaching you to fly is to throw you out of a helicopter. Ilya's classmates at school are permanent comic relief, especially Tatsuko the mental motormouth. Her sports day challenge in the OVA episode is inspired. I howled. However at the same time the threats are real, with magical firepower that might even surpass Magical Girl Lyrical Nanoha and some seriously intense fights. Injuries bleed. The girls' only option is often to run away. The Class Cards are one-man armies and hold absolutely nothing back, being fully capable of killing one or all of our heroes. Ilya's terrified when the reality of all this eventually sinks home. There's lots of comedy and lots of drama, with neither undermining the other.
Incidentally, there are a couple of Magical Girl Lyrical Nanoha references. Ep.7 claims that "Now is the perfect time for our special attack: Lyrical Radical Genocide!" Yup, that's our Nanoha. There's also a two-chapter crossover manga, called 'Lyrical Nanoha x Prisma Illya' or simply 'Nanoha x Fate'.
The lesbian gags are a scream, by the way. Theoretically the show's pushing its luck to be going there with ten-year-olds, but it gets away with it by being so funny that you don't care. Ilya and Miyu get close. Very close. Ilya appears to have an industrial-strength maid fetish, although you may or may not think that's a sufficient explanation for stripping and leaping on top of someone. Meanwhile Miyu in ep.10 is so forward and girlfriend-like that even their schoolfriends comment on it. The funniest thing about all this, though, isn't Miyu and Ilya in themselves, but other characters' reactions, e.g. the misplaced supportiveness of Ruby and Sapphire. "I'm glad to see their relationship deepening."
I love it. Is it my favourite magical girl show? On reflection... no, but I had to think about it. No one might ever trump Usagi in the 1990s anime version of Sailor Moon, while of course there's the argument that Cutey Honey is a prototype of the genre. However I personally prefer this to, say, both Nanoha (despite that franchise's historical importance) and the PreCure franchise (despite that franchise's massive and long-running success). It works emotionally, dramatically and comedically. It's truthful to character, it has heavyweight enemies and it's telling its story properly. Ep.8 hurts. Sella and Tatsuko will be making you laugh at the same time, but the pain remains. Ep.9 will make you scared about Miyu's mental and emotional health. Unlike the deconstructive magical girl shows, though, this is also a fun, happy experience. Or, more precisely, it's a riot.