Any honest review of Nyaruko: Crawling With Love! is likely to boil down to three words: Did I laugh? Well, yes, I did. This is, after all, a story about a kid who finds himself the object of a crush courtesy of an alien whose species parallels the Cthulhu mythos. It's a staple conceit of anime since Urusei Yatsura (broadly influential, little seen), but two things that save it from falling into the same dustbin as most other harem anime: one, it's actually pretty funny, and two, it ranks as one of the single strangest usages of Western source material in anime thus far, bested only by the (probably untoppable) rock-band naming conventions of the characters in JoJo's Bizarre Adventure.
So, the plot, such as it is. "Nyaruko" (aka Nyarlathotep, the "Crawling Chaos") shows up on Earth and saves poor Mahiro from things even worse than she is. The fact that there is a race of aliens whose existence parallels the Cthulhu mythos is not nearly as weird as the fact that they regard Earth as a source of contraband goodies like eroge. That, in turn, bulks quite tiny next to how Nyaruko has a massive — albeit entirely sincere — crush on Mahiro ... but Nyaruko, in turn, is the object of the unrequited affection of her archenemy Cthugha. One loopy bit of hijinks leads to another, culminating in a body-swap misadventure and a teaming-up of everyone concerned to fight off an enemy whose agenda most disturbingly resembles Governor Ishihara's censorship of "media harmful to youth". Hey, I'll take my social satire wherever I can get it, and this is as good a place as any.
I'm always fascinated by how any culture picks up various artifacts of another culture's pop landscape, something that Japan and the West have been doing to each other for centuries on end. That the Cthulhu mythos eventually became part of that process seems more like the logical continuation of a process that's long been in motion, and less like an admission of creative bankruptcy. In other words, this isn't a symptom that the folks overseas have also run out of ideas; it's better seen as an example of someone looking at something that isn't their own and finding a way to make it their own. You can quibble with the end result, but I'm less inclined to see the process itself as a sign of anything degenerate — people are always going to want to mash things up, and so the results should be criticized separately of the original impulse.
What's also striking for me is how the specific ways the show mines this material for laughs is how it echoes the way the more otaku-centric anime titles appeal to their audience by relying that much more heavily on otaku humor, rather than humor that's more a matter of character, situation, personality, etc. Granted, a few of the jokes revolve pretty closely around the Cthulhu mythos per se — e.g., the Call of Cthulhu role-playing game, as when Mahiro comes dangerously close to losing all of his sanity points. But a fair amount of the time it's cultivating yuks that could have come from most any otaku-centric title, as with a whole episode devoted to mocking the XBOX's colossal failure in Japan. The saving grace, again, is that it actually manages to be funny.
The folks likely to get the most out of Nyaruko are those who fall into roughly three camps: existing fans of ecchi / harem comedies, Cthulhu fans with a goony sense of humor (I've found many of them are indeed like that), and lovers of the ridiculous. Put it this way: If Nyaruko's introductory line, "I am the terror that crawls in the night!" is funny to you because you get references to both Darkwing Duck and H.P. Lovecraft, you might well be the target audience.