It's the latest series from Kentaro Yabuki, who also did the notorious nudity-fest To Love Ru. Apparently the manga's pretty lurid (albeit without actually becoming hentai), but the anime's far tamer.
I enjoyed it. It's nothing special and it's strictly middle-of-the-pack compared with other gender-bending anime, but I'm reasonably fond of its cast and I genuinely like some of its story developments. The premise is that Matsuri is an ayakashi-hunting ninja who one day gets turned into a girl by his latest target. Thereafter we're following him and his childhood friend Suzu as they deal with classmates, rival ninja and of course ayakashi.
I should explain some terminology.
YOKAI (YOUKAI) = the best-known Japanese term for weird supernatural creatures from folklore and fairy tales. See the works of Mizuki Shigeru, for instance.
AYAKASHI = basically the same thing. I've heard it said that the term "youkai" is likely to be a physical being (albeit with fantastical features) and "ayakashi" might be a disembodied spirit or an outright apparition, but we're splitting hairs by this point. Sometimes they might appear human, which you wouldn't expect with youkai. It's also a more antiquated term.
MONONOKE = again, basically the same thing, except much more likely to be evil. (I asked Tomoko and her first reaction was "they're the same thing, aren't they?") Miyazaki put them in the title of his film Princess Mononoke.
There are also plenty of terms that specifically mean "ghost", e.g. yuurei, obake, onryuu (those are seriously bad news), but those aren't relevant here. This series has chosen to say "ayakashi", but it could just as easily have said "youkai". Anyway, ep.1 gives us a rough boy/girl (Matsuri) who's determined to keep all ayakashi away from the gentler, all-loving Suzu because she doesn't realise how dangerous they are. She's an ayakashi miko, i.e. her special magical energy makes her delicious. Personally, I found this off-putting. Matsuri was metaphorically insisting that strangers are evil and/or dangerous, even the nice ones, because you can't safely tell the difference between them. Suzu disagreed, but she didn't get a choice in the matter.
...and then, to my delight, Suzu wins the 12-episode argument. She's not just the damsel in distress, to be repeatedly saved by Matsuri. She ends up being more important than him. The show explicitly frames its conclusion as "Matsuri was wrong" (and by this point even he's come to agree) and then underlines that with a confrontation with one of their childhood friends who he drove away for the crime of being an ayakashi. Ultimately, she saves him.
The most dramatic symbol of this is Shirogane, the King of the Ayakashi. He wants to eat Suzu, so Matsuri wants to kill him... and Suzu protects Shirogane, even though she's protecting a shameless man-eating monster who openly declares that he intends to eat her at the first opportunity. (They turn him into a cat and look after him. That's a pretty cool triangle in itself. I love Shirogane. He's our heroes' openly evil best friend, helping them if he feels like it but always insisting that he's their enemy and stubbornly refusing to admit that he might be thawing towards them.)
Another triangle involves Shark Boy. (He has a name, but never mind that... he's Shark Boy.) He's Matsuri's intense macho rival and will acquire a crush on Suzu. I was waiting for the character to get tiresome, but he never does. He behaves reasonably, especially given his personality issues, and I admired his stance in ep.12.
This is a fanservice series, with censorship in the TV version and nipples in the Blu-rays... but the characters and storyline aren't really tailored to that kind of thing. If someone described it to you in outline, you'd assume this was a regular show. It's about ayakashi. They usually seem harmless, but a few are genuinely dangerous and that's what Matsuri and Shark Boy are absolutely focused on. Matsuri prioritises Suzu's safety above regaining his original gender and indeed preserving his life. No one's a horndog (except you count Yayoi groping her friends in the early episodes and the show soon ditches that). There's also an evil loathsome sadistic bastard towards the end and our heroes' fight against him is heavy-duty.
Stuff I didn't like... the panty shots get tiresome. They overdo Suzu's catchphrase of saying "mouretsu" (meaning intense, violent, etc.) to mean "very". I also disliked Matsuri's habit of not telling Suzu things (including one thing that's a big deal), but Matsuri is dysfunctional and didn't expect ever to have friends. He's got a lot of growing to do too.
This isn't a particularly great anime. I quite enjoyed it, but I don't know if I'd call it a keeper. It's often a bit silly and its supporting cast tend to be lightweight. I liked Matsuri, Suzu, Shirogane and Shark Boy, but no one else really matters and you'd take the show more seriously if it didn't keep returning to boobs and panty shots. Fundamentally, though, I genuinely like the core story it's telling and I'm glad I watched it. (I'm not convinced Season 2 would live up to it, though, if they make one. We've seen the important character journeys. Where next? My prediction is lots of filler episodes and storytelling that's treading water.)