Mediocre results.
*New words you add to dictionary are never offered as correction.
*Many times there's no proper correction for simple typing mistake, like changing the order of two letters in a word - the suggestions are weird and irrelevant.
TL;DR - barely more useful than Firefox' (horrible) built-in speller.

It's a very useful tool for spell checking but I'm very sad that it's not available in Hungarian. I know it's a difficult task to include all languages but I still expected all EU countries to be available. Disappointed.
Still, the app works as advertised when I'm using English.

В целом хорошо. Но попадаются обычные слова которые не знает, отсутствуют в словаре. Большим недостатком является отсутствие проверки на букву ё. При этом блокирует встроенную проверку от браузера, где есть проверка на ё. Поэтому вынужден отказаться пока от данного расширения.

It's okay, but I have a bug where it doesn't check commas and punctuation.

And also on some form fields (like this review) it just doesn't work at all.

Until recently, it was really great, but since the last update, it's become quite annoying. Every time there's a suggestion for correction, two windows pop up simultaneously, which you always have to close with a mouse click, and that's killing my nervs. If you, like me, use a lot of specialized terminology that the language tool doesn't recognize yet, the entire text is constantly plastered with suggestions. It makes working impossible. Additionally, it now only wants to correct into Dutch, even though German and English are selected. This renders the tool completely unusable.

Mostly works well, but lately it puts two dialogue boxes, one of which does not work.

Wenn es nicht oft die falsche Sprache erkennen und dann alles als falsch markieren und sinnfreie Vorschläge machen würde oder man die automatische Erkennung ausschalten könnte, wäre es richtig gut. Ich habe Muttersprache Deutsch und bevorzugte Sprache Deutsch und dann kommt es mit Niederländisch daher und macht alle Vorschläge in Niederländisch.

Nachtrag zur Antwort.
Danke, das habe ich schon mal gemacht, brachte aber keine Lösung. Das Problem tritt vorwiegend bei Google Notizen auf und selbst wenn man händisch auf Deutsch stellt, ist es nächstes mal wieder Niederländisch. Am besten wäre wohl, die automatische Erkennung auszuschalten.

Just started using it. We shall see.

Excellent product. However, not satisfied with the add-on. The inline pop-up closes every time the page is scrolled. My display is 1280x, and I use a mouse, and i will have to scroll to see the language suggestions. It becomes very annoying to reopen often. Need an option to close manually only using the close button, it should not close on page scroll. It also sometimes closes itself, when fetching paraphrasing suggestions, must be a bug.
Update: Posted, and pending.

After 2 years, it mostly cannot detect and disable checking when writing in unsupported language (Czech).

I would rate 5 stars but I think you should be able to turn of the notification that the text has advanced errors. I understand that they need a subscription of that but I think we should have the option to not accept, I would rather it just ask me if I wanted to upgrade every once in a while rather than always tell me that there is a problem and not telling me how to fix it.

It works nicely, but since the 13th of July update, I have a constant "!" warning when I write a text and I have to click it then click "I understand" in a separate page like 10 times a day, I don't know why it was perfect before...

It will work almost all the time when correcting your spelling, but many times it misses the grammar, hence not as polished as Grammarly.

Example:
OK, so I write the text, and it will correct me attempt (this sentence is correct according to LanguageTool

However, according to Grammarly it corrected my text to (OK, so I write the text, and it will correct 'my' attempt) which, I believe, is better.

There are many more cases where LT cannot detect the proper grammar. Also, I am on premium plan with ''Enhanced grammar, punctuation and style checking''. The only reason I use it because it helps me with additional language. Overall, it is decent for the job, but if you speak only English, then I don't recommend it.