Since I read a few threads discussing it when I was looking for stuff, I might as well write a rant about it here too. Manga or anime (with Japanese subtitles; not with sound-only, because you won't catch everything) is AMAZING for learning/practicing even N1 grammar. People (English-speakers) online say differently, however.

Grammar that people claim is "nuanced and never shows up in real life" shows up constantly in anime, and fiction in general. When people say "anime will teach you to speak weirdly", what they're thinking is that you're not literally going to say the full sentence "I'm totally gonna kill you!" in real life. They for some reason think that all your Japanese is going to be 100% parroted sentences and that you can't/won't make up anything on your own. What they don't realize is, you actually ARE going to say "I'm", "totally", "gonna", "kill", "you" in various different sentences, as well as the sentence structure "I'm (totally) gonna (verb)", etc.

They also tend to say your pronunciation will be "wrong", but sounding like "Japanese TV meant for Japanese natives" is a billion times better (and easier for Japanese people to understand) than sounding like a foreigner. As is with any language. I might laugh if you speak English like Action Movie Narrator Guy, but it's way better than having an accent so thick I have no clue what you're saying.

Here's one place where I'm grabbing grammar from:
http://www.tanos.co.uk/jlpt/jlpt2/grammar/usage/
http://www.tanos.co.uk/jlpt/jlpt1/grammar/usage/

Out of the around 250 JLPT N2 grammar points, almost half of them were completely redundant or obvious & not actually separate meanings (ex. 上は、上で、上 are obviously all the same; the meanings of から見る and ことなく are obvious if you already know the individual word meanings). Out of those, some of the ones I can say — without even thinking about it — are really common in anime are:

N2 and below: ALL of them.

N1: いかん, かぎり, 極まる, すら, おかない, のみ, やまない, 至る, おかげで, etc. 1/3rd of the N1 list is super common. At least 1/3rd isn't so common but they still show up enough that I remember seeing them (ex. I think といい has only shown up once in 50 episodes, but I remember it. It could also just be that my particular genre of anime doesn't use this much).

Then there's the vocabulary. It's a LOT easier to learn and remember words you'd never otherwise use, when you learn with anime/manga. This is because out of nowhere you're going to get "crime suspect", "senate" and "cucumber" even in series that should have nothing to do with the police, politics or cooking; so instead of trying to tackle, say, a newspaper article with a billion political terms all in one go, or an archaic story all in one go, you're instead fed the unknown terminology little by little, series by series, character by character, until suddenly you realize, waitaminute, I can read the newspaper with ease! On top of that, manga normally gives you enough context clues (pictures, other characters' lines...) that you can figure out a lot of unknown words just by context alone, which is not something you can normally easily do with novels or other types of text. That and well, the situations and sentences are more memorable in fiction, so you have a higher chance of remembering the grammar and vocabulary. It's "Half the members of the senate are actually parasite monsters! Roll up your shirt sleeves, it's time to battle!" versus "The senate passed a bill today".

The same is true for politeness levels. It's obvious but, in anime and manga there's a huge mix of politeness levels and dialects compared to what most foreigners see in normal everyday life. I know people who've been living in Japan for more than ten years who still mix up even the basics of politeness levels when they talk / write. I've seen people who've been living in Sweden for years and who still can't understand Swedish dialects. This is because they don't ingest any kind of media.

In anime/manga you have "that one princess character who speaks super politely", "that one punk who speaks super rudely", "that weird kid who speaks in dialect", "that annoying nerdy guy who talks like a samurai". You're getting 3 different levels of politeness and 2 different (watered-down) dialects all in the same show. There's not exactly any effort needed in memorizing politeness levels because you already have the stereotype of what kind of person says that stuff stuck in your head. I mean, seriously, people's argument about "don't learn from anime!!" is that they think you're so stupid you're literally going to say "You piece of shit!!" to your boss, because you don't realize it's an insult. That's impossible, because guess what, anime/manga shows not only the type of person who'd say that and what kind of situation they'd say it in, but also the (angry) reaction of the person hearing it. Frankly that kind of mistake is much, MUCH more possible if you're NOT watching anime.

So I have a quick example here. One person said they've only ever seen x grammar from the N1 list, in advertisements (where it's all over the place). Guess what. There are advertisements in anime/manga. I don't just mean literal adspace squeezed in next to the manga panels, I mean you see billboards, flyers, commercials etc as part of manga plots and you also meet characters who literally talk like advertisements (Car Salesman Guy).

You know what you also get in manga? HANDWRITING! The average learner of Japanese can't read normal Japanese handwriting because they're only taught the "proper" way to handwrite things, not the shortcuts and messiness that Japanese people actually use. But in manga and doujinshi handwriting's all over the place!

Manga also uses normal words as sound effects; ex. "slide-slide", "gloom-gloom", "tremble-tremble", "sparkle-sparkle" or whatever. According to my teacher, in actual spoken Japanese these are just said in sentences like any other normal word ("My head was thud-thudding") but in manga they're more often standalone words used to describe the actions in the panel (person holding their head: thud-thud), so you can more easily figure out what they mean and when to use them in manga, compared to in other types of media. The same is basically true for phrases and proverbs, for one reason or another in manga it's just often much clearer what the phrase means (again, says my teacher).

There's also the kanji side of things. Manga (and by extension, anime subtitles) use a LOT of different kanji and are really dependant upon you knowing the individual kanji meaning and/or pronunciation because they constantly make up new words, make up puns, write older or stranger kanji to slightly change the nuance or meaning, or even show the kanji word with an English katakana definition (ex. 悪魔 in kanji but デーモン as the furigana on that kanji). Maybe an attack name is "Iron Claw", or the last name of the albino guy is "White Hair". In nonfiction you're used to seeing these things in the same words over and over, so you can end up recognizing the entire word and not the individual kanji. You're not going to get anywhere near this same amount of practice if you're just reading a nonfiction book or your textbook.

Speaking of, after studying from textbooks I can tell you that the same grammar points that show up maybe 5 times in the average textbook, show up 50 times in quick succession in anime and manga (and often with much clearer example sentences). No matter how bad you are at grammar, you've got to have some kind of guess at how to use it after seeing it 50 times in different situations.

...Well, if you can tell, I'm really fed up with people claiming you can't learn or will learn wrong from anime/manga. I really wonder what in the world these people who are against it, are trying to learn Japanese for. I mean, they're obviously not learning for the sake of watching TV, playing games, reading books or listening to radio dramas — you know, the kinds of things every single native speaker does every single day. "They don't like fiction" isn't even a valid point, because there's tons of nonfiction manga.

By the way, for some reason a lot of these people are for learning by dramas and general live-action stuff, but against learning by anime/manga. This makes zero sense because the dialogue, plot and actions (character movements — I mean how the characters literally move) in those is usually exactly the same as in the worst anime/manga.

This was all just my opinion after 1 1/2 years of learning Japanese and a lifetime of "don't learn from anime/manga!!".