With English being my second language I find Grammarly absolutely essential. I dabbled in creative writing in my native language, was a bit proud of my skill and made it a point to use correct grammar, punctuation, and spelling in all kinds of written communication, even on IMs.
To get to this point, I did primarily two things: I read a lot, and I wrote a lot. I then submitted my writing to my peers, similarly interested in creative writing, who would also -
in addition to the story, characters and so on - criticise my word choices and my grammar. It took years, but it was enjoyable, and I acquired my second (and last, after programming) skill that could be of some value.
A couple of years later, when I started working, I had to switch to English. It wouldn't be that big of a deal were it not for my experiences: every time I had to write anything, I felt incredibly constrained, like I'm missing half of my brain. I was used to being able to express myself precisely, clearly and elegantly in writing - all of that stopped working after the switch. It's incredibly frustrating, to the point that for a few years I was in complete denial and refused to write in English wherever I could get away with it.
Well, I thought, I got proficient in Polish, so technically I should be able to get to the same point in English, right? It's easy - I just need to read a lot and have a group of people who'd like to read my writings and correct my mistakes. Easy!
...however, I'm not in high school anymore. Between work and the little social life I have, there's not that much time available for pursuing other matters. I do read a lot, exclusively in English, but these are mostly tech-related articles, blogs, and books, written by people who couldn't care less about beauty and elegance of their writing. It's actually counterproductive if my goal is to get better at writing - such posts are chock-full of both errors and merely weird wordings and constructs. And nobody seems to care.
There is a creative writing StackExchange (and many other places), where I could submit my texts to get the criticism and corrections I need. Unfortunately, I don't have the time - even if I had the skill - to do my part of the deal, that is, to read and comment on writings of others. I'd feel bad exploiting strangers like that.
As you probably already guessed, this is where Grammarly comes up. It gives me a bit of the feedback I need to improve my writing. It's not at the level of other humans, which is obvious, but it does catch some mistakes and some stylistic problems. It doesn't rely on unpaid work of others, so I have no qualms about using it. I'm not worried about following its advice because even if it's wrong, nobody would care. The amount of contempt for the language in the tech community is staggering; average tech-related writing is on such a level that I'd rather chop my hand off than write like that, but it shows just how unimportant correctness and elegance is for people (as long as it gets the point across... right?)
So, to summarize and get back to the topic at hand: Grammarly is non-ideal on so many fronts, that to simply enumerate them would take until Friday (it's Wed today). But it's also the only tool I can rely on, and it does an acceptable job at what it does. It breaks web pages, it's unusable from outside a browser, it's error indicator is frequently displayed 3+ lines from where it should be, it's stupidly dumb and cannot, by itself, tell where the additional "actually" is actually needed, but it's the only help I can get, so I use it.
Of course, if your goal is merely communication you do not need anything other than basic spell-checker and a book on basic grammar. On the other hand, if your goals are similar to mine, then Grammarly is one step above that combo. It isn't, and probably won't ever be, anywhere near the level of human reviewers, but it is something.
PS. For a long time, I wanted a feature in Grammarly that would automatically slurp content of blog posts and articles in, so that I can just click a couple of times and then read the post without all the mistakes and weirdness people so often put in there. Reading the top-voted comment here, about the license, I see why they won't implement it. Similarly, I guess Emacs plugin is not going to emerge anytime soon. Whatever you use Grammarly for, you should assume it's public. It doesn't make it any less usefull for writing comments on some sites or posts for my blog, though.