I usually despise harem anime.
I feel I should make that clear from the outset, since I'm about to explain just why I loved Nyarko-san so much.
Now, there are a few harem anime series I've enjoyed. I love Ranma 1/2, one of the originators of the genre, though I'll concede that after a while it got tiresome. I also really enjoyed the first season of To Love-Ru, but that's mostly because the harem stuff was balanced out by quality fanservice, great character designs, clever slapstick, sheer weirdness and the author and staff's apparent fetish for completely gratuitous tentacle scenes, a fetish that I cheerfully admit that I share.
Most other harem anime can take a flying jump as far as I'm concerned. I've gotten so sick of seeing the same cliches used over and over (the Bad Girl and the Polite Upper-Class Girl have a rivalry, there's a Loli Girl with a not-at-all-creepy crush on the Main Guy, the inevitable cooking contest, etc.) that I usually avoid them like the plague.
So why did I decide to give Nyarko-san a chance?
Because the premise was too specacularly weird not to check out.
So there's this Ordinary Japanese High School Student. His name's Mahiro, and he's voiced by Eri Kitamura (Madoka Magica's Sayaka Miki! Yes, really!). Some hideous demon thing chases him and corners him in an alley... and then gets brutally bludgeoned to death by a silver-haired girl armed with what appears to be a crowbar. She introduces herself as Nyarlathotep, "the chaos that crawls up to you with a smile!" Cue insanely infectious opening credits. (Un! Nyan! Un! Nyan! Un! Nyan! Let's NYAN!)
Yes, she means that Nyarlathotep, as in H.P. Lovecraft's Elder God. Turns out all the horrifying beings Lovecraft wrote about were based on visiting aliens who came to Earth to try to experience human culture... Apparently our movies, TV, and games (particularly porn and otaku bait) are a hot commodity in the worlds beyond. This particular Nyarlathian, "Nyarko", is a hopeless fan of mecha anime and toku shows (responding to one that's basically Kamen Rider in all but name: "GASP! A Kurogane no Striver BLU-RAY BOX! And it comes with a limited edition figure!" Kana Asumi's delivery of this line cracks me up every time. It's something about the way she says "BLU-RAY BOX", I swear it's hilarious.)
As a satire of moe-anthropomorphosism, this is already pretty great. We've turned nearly everything else into doe-eyed anime beauties, why not Elder Gods? But what drew me into Nyarko-san even further than that was its cheerful eagerness to make fun of itself and the harem genre, taking pains to point out how contrived everything is. Nyarko's initial explanation of her origins gets fast-forwarded by the main character, who just wants to get on with it already. Mahiro's a veritable fourth-wall-breaking machine, never failing to point out when the plot isn't making a lick of sense.
Something else refreshing about Mahiro: finally, finally we get a male character in a harem anime that isn't a dithering, indecisive moron when it comes to women. Mahiro knows almost from the outset that despite Nyarko's cute, bubbly, teenage exterior, there's an indescribably hideous alien underneath, so sleeping with her would almost certainly be a very bad thing. This actually gives him a reason for the dusty old "constantly brush off the would-be love interest" trope. Whenever Nyarko gets too gropey, he stabs her with a fork... which becomes a running gag.
I might be completely failing to do justice to this series. Repeatedly stabbing a lovestruck teenage girl (or something that looks like a lovestruck teenage girl) with a fork is bound to turn some people right off this show, but for me it's a breath of fresh air. Most harem anime are rigidly locked into a formula established decades ago... Nyarko-san takes the formula and smashes it with a crowbar, excuse me, "unspeakable bar-like thing". It's gleefully weird and insane, it's surprisingly cute at times, it's filled to bursting with references to anime and toku, and it pretty much never takes itself seriously. Oh yeah, and the writers obviously really, really know their Lovecraft... if you know what to look for, it's surprisingly faithful to the mythos, and little nods to it show up everywhere (the best of which might be the "Cola of Cthulu" Nyarko is seen drinking in one of the eyecatches, though the depiction of the Sunken City of R'lyeh as an equivalent to Disneyworld is also pretty genius... "Look! It's Mickey Innesmouth!")
So, in conclusion: most harem anime are garbage. This is one of the rare ones that at least attempts to do something different, and for the most part it succeeds wonderfully at that. I eagerly look forward to the upcoming second season.