It's a middle-of-the-road isekai. Ordinary bloke gets summoned to a fantasy world where he has amazing superpowers, can crush even the most terrifying bad guys in his sleep and soon has a harem fawning over him. I quite enjoyed the early episodes, but by the end I was skipping through.
It's not bad, though. It's usually likeable. Our hero is a game designer who's been sucked into his own creation as Hakuto Kunai, a tall, black-clad man with a bad cigarette habit and a voice like a growling dog. He looks and sounds villainous and can kill demons, so he gets called "Demon Lord". In fact, though, he's the kind of nice guy who'll even remember to be considerate towards the coach driver that even the audience will have forgotten about. He's thoughtful and sharp. Underneath his extremely dodgy traits, he's lovely... but he has a habit of looking and sounding evil, which is entertaining.
The show's pleasant and nice. No one means ill and it's a relaxing series to dip into. It has three problems:
1. Lack of drama, although that's normal in this kind of isekai. Nothing can even slow down Kunai and the fight scenes are just an ego stroke.
2. Kunai's goal is to build a hospital, hot spring and public bath. Okay, yes, that's not an illogical direction for the series given the previous point, but it still means you'll end up watching Tourist Resort Management: The Anime.
3. Some characters who tested my patience. This show's supporting cast tend to have serious personality problems and/or sexual issues, e.g. a little sister complex or being into little boys. Luna Elegant is a selfish, bratty Holy Maiden who keeps accusing Kunai of being obsessed with her arse. Yu Kirino is supposedly Hannibal Lecter in a skirt, although she only ever does one dodgy thing and that's to a would-be assassin. Tron is a demon, etc. Those are the okay ones, though. That lot I didn't mind. Sorted in order of my personal pain, the ones who made me want to stop watching were YUKIKAZE (a boy who looks like a girl and can't stop talking filth), ZERO KIRISAME (macho show-off alternate persona of Kunai, who finds him embarrassing too) and to a lesser extent KILLER QUEEN (who likes Zero) and that cliched camp dressmaker.
Aku's adorable, though, and Angel White's quite a good character. Even after I'd basically given up on the show, Angel White rescued ep.12.
Kunai acquires a harem and doesn't take advantage... but this is plausible, given his Demon Lord persona. He talks to everyone like a yakuza. The only person he'll find attractive is Angel White. Even when girls (Aku, etc.) climb into his bed, it's like father and daughter and it's obvious that nothing happens.
The show also avoids fanservice, despite ultimately becoming a harem show in a hot spring. That said, though, Mikan's bikini pseudo-top wouldn't reach the nipples of 90% of women. There's also a race of bunny girls.
Essentially, I like this show. I'm fond of the cast, despite some glitches, and in theory I don't even mind Kunai's business plan. However the show's sidelining its own interesting story elements, e.g. the evil tempting ring in ep.3. Nothing happens with that. I could say something similar about the Satanists. If you watch this show, you'll probably skip a few episodes towards the end and not feel you've missed anything... but it's still amusing. I like the way Aku almost comes to regard "sinister and menacing" as Kunai's charm points, for instance.
This show is junk food, but it won't poison you or anything. It's harmless.