It's hilarious and I loved it. The last three episodes are perhaps slightly weaker, feeling the need to build up to a dramatic finale with a fight against a big unstoppable monster. However it's still a ton of fun and I'm looking forward to the promised Season 2.
It's a Flash-animated comedy where all the cast are back in high school for no reason. (They're aware of this, complain about it and try to analyse it.) What's different is that it's a crossover between four Kadokawa anime: Konosuba, Overlord, Re:Zero and The Saga of Tanya the Evil. All four are "isekai" shows, i.e. with a protagonist from the real world who's been dragged into another reality, usually a RPG fantasy one. You should watch them all. Apart from anything else, you'll obviously enjoy this show more if you get all the references. The four parent shows are:
1. KONOSUBA - the odd one out because it was already a parody/comedy, whereas the other three shows were serious. (It also has low-powered and/or incompetent anti-heroes who are just ordinary adventurers, whereas the other shows' casts are much more high-powered.) Kazuma is a lazy, perverted dirtbag with no discernable morals. Aqua is a stupid, useless goddess who thinks she's wonderful. Megumin is a witch who refuses to use any spell except Massive Scary Explosion. Darkness is a crusader who can't hit anything with her sword and whose only use in combat is to suffer pain... which she enjoys. A lot. Darkness is the most hilariously appalling character in Isekai Quartet, which is saying a lot if you look at the competition.
2. RE:ZERO - the odd one out because it's quite a harrowing, challenging show that doesn't match the tone of Isekai Quartet at all. It's impressive, but I wouldn't call it fun. Subaru is a hikikomori whose only special power involves getting killed over and over again. This lets him rewrite a small section of reality, but the consequences of this tended to be disastrous, tragic and/or sanity-shattering. That said, though, the Re:Zero cast work better here than I'd expected. They have cool character designs (especially Rem and Ram) and they've got enough quirks and powers to stay visible in a huge cast of rampant scene-stealers.
3. OVERLORD - completely belongs in this show and isn't any kind of odd one out. Ainz Ooal Gown is an evil undead overlord (but underneath a nice-ish guy) who's trying to conquer the world with the help of his super-polite but staggeringly evil minions. They're villains. Show them a village full of innocents and they'll turn it into a burning crater of corpses. The rest of the Isekai Quartet cast is only still alive because attacking other students is prohibited by the magic power that brought them here. Ainz's servants include Demiurge (a superintelligent devil who makes scroll parchments of human skin), Albedo (a psychotic succubus who's more evil than Demiurge), Shalltear Bloodfallen (a vampire) and Cocytus (a samurai insect the size of a fork lift truck).
4. THE SAGA OF TANYA THE EVIL - the odd one out because Tanya's world is basically World War One with magic, whereas the others are set in "orcs and dragons" Tolkeinesque realities. Tanya is a little girl, although she used to be a human resources contractor in Japan before dying and getting reincarnated by God (who she calls "Being X"). She's also close to being the most evil character in Isekai Quartet, although on the upside she's also a military disciplinarian who obeys orders and dislikes messing around. She'll befriend Ainz and Demiurge.
I loved it. It made me laugh all the time, starting with the theme song. (I love in-character theme songs. This one's sung by Kazuma, Subaru, Ainz and Tanya, while the closing theme is shared between the shows' leading ladies. Tanya's in both.)
There's not much here to discuss or analyse, because the show's almost never serious and my favourite bits are all jokes. (I did say "almost", though. I liked Emilia's quiet scene in ep.7 where she observes that "no one calls me the silver-haired half-elf here".) Almost everything I'd want to mention is just a scene that made me laugh, so this review is about to become mostly random observations. That said...
(a) relative power levels might not be what you'd expect. Tanya appears to be immune to Ainz and Demiurge's magic spells, while being capable of unleashing a battle aura that's compared to an earthquake and is said to exceed an 8th Tier Spell by Ainz. Then there's Aqua the useless idiot, who also happens to be a goddess with anti-undead powers. She could probably trash half of Ainz's army solo.
(b) Ainz is surprisingly funny. It's not so much the contrast between his inner and outer voices (nerd vs. DEEP EVIL OVERLORD), although that can indeed be amusing. It's more that Ainz is a skeleton with no facial expressions, which makes him perfect for comedy micro-reactions when Demiurge or Albedo say something he wasn't expecting. (It's also amusing to see him in a swimsuit on the beach in ep.9.)
(c) the Konosuba gang could make me laugh just by opening their mouths, but they also become subtly less prominent as the season continues. The other three shows' casts can find common ground. You can have a sensible conversation with them. They're all capable of playing the straight man. You get surprising friendships like Tanya-Ainz or Albedo-Rem (aka. the Implausibly Fast Falling in Love Association). Kayuza's mob, though, are irredemable trash who squabble non-stop, albeit mostly with each other, and they don't really make friends. Enemies, yes. Aqua hates the undead. The only Konosuba regular who doesn't fight with anyone is Darkness and that's only because she's such a depraved masochist that fighting would be preferable.
I still love them, though. I want more Konosuba.
(d) I never knew Albedo was a succubus! (Do succubi usually have buffalo horns?) I always just thought she was depraved (and scary), although immediately afterwards we learn something even more surprising about her.
(e) Tanya finally realising that Aqua is indeed a goddess in ep.10... heh heh. Did I mention that she wants to murder Being X?
It occurred to me to wonder if any of the voice actors did double duty, incidentally. There were some. Rie Takahashi plays Megumin (Konosuba) and Emilia (Re:Zero), while Yusuke Kobayashi plays Subaru Natsuki (Re:Zero) and Warren Grantz (The Saga of Tanya the Evil).
It's great. Strongly recommended, but only if you watch its parent shows first. I also can't help wondering about the transfer students that we've been promised for Season 2. Looking just at other Kadokawa franchises... And You Thought There Is Never a Girl Online? wouldn't fit, though it might be funny. Danganronpa would make my head explode. I bet it's Sword Art Online, but what about Ringu? You know, the J-horror live-action films...