It's the crudest, most vulgar anime I've seen to date. Akutabe is a demon summoner who runs a detective agency and uses demons as his employees. He's scarier than any of them and will kill, mutilate, decapitate or otherwise inflict ultra-violence against any supernatural being who displeases him. (They're immortal-ish.) As for the demons, they're all loosely based on real Christian mythology but take cute animal forms in our dimension. (Akutabe's cast a spell on them.) The chasm between their badass real identities and goofy Earthbound ones is one of the show's better jokes.
1. AZAZEL, a dog-like demon who can induce uncontrollable lust in humans (or make them unattractive and/or impotent), but mostly just wallows in perversion and sexually harasses Akutabe's assistant, Sakuma. Loves pornography. Is functionally useless under almost all circumstances. Frequently tries to betray or backstab his friends.
2. BEELZEBUB, who looks like a penguin but is really a shit-eating fly. Literally. He might warm it up in your microwave. He's the most intelligent, polite and co-operative of Akutabe's demons, but that's a low bar. He's less likely to push you under a bus, but that's still where you'll end up. He can hypnotise you and reveal your deepest, darkest desires. Or else make you shit and/or piss yourself uncontrollably.
3. UNDINE, a clingy mermaid and a massive pain in the arse. She'll decide that you're in love with her, then scream at you for not doing the opposite of what she says. She's gullible. She's jealous. If she thinks you're happy when she's not, she'll burn with rage and rewrite reality to turn you into a monster.
4. KIMITAKE SALAMANDER, a parody of ultra-nationalist author Yukio Mishima (who raided a Tokyo military base in 1970 and unsuccessfully tried to persuade Japan's Self-Defense Forces to overthrow the pacifist post-war constitution. He then committed seppuku after screaming "Long live the Emperor!".) This anime's version is a masochist and an outrageous comedy misogynist. "A woman is a creature akin to a pet. Men are meant to take them out on walks."
These people are appalling. Everything they do goes wrong, usually because someone was gleefully doing things they shouldn't. All the clients who come to Akutabe in Season 1 have their lives wrecked. (He doesn't have any clients in Season 2, which is slightly less cruel than its predecessor.)
Angels are also repellent, though. They walk the Earth looking for demons and will ignore anything that humans do to each other, even if it's right under their nose. As for God, he's a silhouette who makes juvenile shadow puppet jokes.
There's one likeable-ish normal character. Rinko Sakuma is a sensible girl who works for Akutabe and shares our opinion of all these demons... up to a point. She doesn't quit her job there, after all. She's a demon summoner just like Akutabe (albeit not as skilled as him) and I wouldn't recommend putting your life in her hands either. (There's a funny moment in the third OVA when everyone's pulling happy evil faces because they're about to do something bad... and Sakuma's among them.)
2010 OVAs (2 episodes) = don't start watching here! They were released first, but they don't tell the beginning of the story. They were bundled with limited editions of the manga's 4th and 5th volumes, so they're just part of things so far. I don't know if the first one even fits TV series continuity.
2011 SEASON ONE (13 episodes) = our heroes assist a stalker, generate urine rainbows, use a chainsaw on each other and give everyone at a comics convention food poisoning. (If you can call it food. Beelzebub was involved.)
2012 OVA (1 episode) = wow, Lucifer's full of himself.
2013 SEASON TWO (13 episodes) = there's a haemorrhoids storyline that goes even cruder than you're imagining and ends with comedy gay rape. In case you were under the impression that this show had even the loosest connection with taste. Sakuma gets taken to Pervertland. Azazel's lecher powers turn out to be useful, astonishingly, but then we discover that he's even more of a loser than we expected.
2014 OVA (1 episode) = crude beyond belief. And they're total scumbags.
Is the show worth watching? Not necessarily. Sometimes it's a bit boring. It will, though, blow your mind with how childish, vulgar and depraved it's possible even for anime to be. And I've watched a lot of anime. Sometimes, I'll admit, it made me laugh.