I quite liked it. That said, it quickly becomes an exploitative "heroes vs. demons" harem boobfest in which the main character is regularly having sex with the girls. (No penetration, but I'd still call it sex. How else would you describe two naked characters pleasuring each other with multiple orgasms, nipple sucking and sex aids? Household implements, food items...)
That sounded fine to me, though. Sex is a normal human activity. People do it in real life. There's nothing implausible about it, whereas most anime that want to be dirty will instead tie themselves in knots with stupid plot points and/or contrived characterisation. Such anime might have, for instance, girls vying to throw themselves at a personality-free boy who never notices or takes advantage, clothing that can't stay on for even ten minutes, conversations always taking place for no reason in the public baths, etc.
Personally I'd call straightforward fornication far less offensive. Characters can be themselves and explore the psychological angles of the situation more honestly and directly. I'm happy with that.
That said, though, this show is indeed very very ecchi. There are long, super-explicit sex scenes (given the no-penetration proviso) that will often change direction halfway and become even kinkier than you'd been expecting. Maria is a nuclear-powered inappropriate behaviour generator, while Yuki regularly blew my mind. (Did I mention that Maria's underage, by the way? Fortunately Basara doesn't want to diddle her, despite all her creative ideas to make everyone's lives more interesting.) There's an occasionally worrying submission-dominance theme of which I'm not a huge fan. There's also a family relationship angle, with the main characters seeing themselves as siblings (and a metaphorical mother-substitute in the OVA). Siblings shouldn't do that together. Mothers and sons shouldn't either. Hmmm. Well, they aren't blood relationships.
I was watching the uncensored DVD episodes, by the way. The censored TV version can hardly be watchable, surely, but what's more the DVD episodes are longer. They've added fourteen minutes of extra material. That's over a minute an episode, on average, which I bet is almost all hentai. Compare with something like Everyday Life with Monster Girls, for instance, which is also nipple-ridden on DVD but hasn't changed its running time.
Anyway, the plot. Basara Toujou is a teenage boy whose father wants to marry a woman who lives abroad. He'll be losing a father, but gaining two stepsisters! These are Mio (his age, huge breasts) and Maria (younger, filthy-minded). This is kind of repellent for half of ep.1, but then comes the twist.
Mio is a demon princess and the heir to the powers of the Demon Lord Wilbert. (Greatest Name Ever.) Maria is her succubus retainer. (Which explains a lot.) Meanwhile Basara is a hero with superpowers and a demon-killing heritage. (Yes, dad knew exactly what he was doing when he dumped Basara in this situation.)
This premise is a lot of fun. Mio and Maria are actually pretty fearsome fighters and don't need help against anything but the highest-level supernatural enemies... but those are the kind of enemies they'll be attracting. Oh, and Basara has an impassive childhood friend, Yuki, who appears to be immune to jealousy but doesn't want to get left out. Yow.
Ep.1 is comparatively clean, for what it's worth. This show might have been family-friendly if it hadn't been for Maria. It doesn't take her long (ep.2) to suggest creating a magical master-servant bond that will: (a) allow Basara and Mio to be able to track each other telepathically, and (b) make them stronger fighters. Sounds good, right? However there's an X-rated catch, because Maria's a succubus. This makes for an unusual relationship in which Basara and Mio will soon be having regular sex (ish), but for entirely practical reasons that don't exclude, say, a third party from also establishing a master-servant bond. They establish a close, warm relationship, but it's one that they see as siblings (eh?) even though Mio's manner can suggest unsisterly feelings if she sees Basara in a compromising situation. (She's not stupid, though. She always knows that the one who deserves her wrath is Maria. You know, the one who's probably filming it with a video camera.)
The family thing is one of the more appealing things about this show, incidentally. Demons can have had their parents murdered too. Having a place to call home is actually a big deal for Mio and Maria.
The other surprisingly thing about the show is the fact that it gets serious. Killing is on the table. What happens to the villain in ep.12 is slightly shocking, which I admired. People can be strongarmed into betraying their friends. The last few episodes build up to something fairly epic.
The only thing I don't like is that dominance thing:
"Strong-willed women tend to be masochistic."
"Please be rougher and make her submit."
At its very worst, this can look as if it's flirting with being rapey. I presume this is deliberate, for supposed audience appeal. Rape is a porn genre in Japan. Some of Basara's more forceful actions can look a bit off-putting, while ep.2 has Maria making Basara perform sexual acts on Mio against her will. Sort of. Is that scene consensual? It's complicated enough that I was able to keep watching, but... yeah, well.
Apart from those undertones, though, I liked this show. I quite enjoyed both the characters and the gradually unfolding drama. It's also capable of genuinely funny sex comedy. Ep.6 made me laugh, while the OVA episode at one point manages to be extreme enough not to be pointless. (OVA episodes often don't manage this.) You'll have to accept a massive level of nudity and explicit sexual content, but I actually quite respect the fact that they're portraying active sexual relationships and in the process going further than some hentai. It's... crumbs. Gosh. It's still borderline pornographic, obviously, but in a good way.
Well, unless you're watching the TV version.