As far as I know they still send everything you type to their cloud service, and their privacy policy gives them broad latitude to use that info to "improve the service."
Right now I use "LanguageTool" with the Java-based server running locally. After I imported the ngrams it works good enough to be a clear improvement over Firefox's bad built in spelling/grammar checker. The UI is passable. It does randomly decide I typed another language though and tell me everything I typed is misspelled in that language.
In an ideal world I'd prefer to pay LanguageTool money for their premium product, but they have the same privacy problems as Grammarly. Heck I'd pay Grammarly if they had a more private offering, it is a good idea.
In a 300 word email I make at least 3-4 spelling errors either because I don't know the words spelling properly (yes there are many words) or because I type fast make mistakes. Grammarly checks their database and compare my sentences and suggest different ones. Try it, it really does help.
Out of curiosity, is English your first language? I can see that it would be helpful for fixing the types of errors you mention, and the kinds that appear in your comment. Does it give suggestions for text boxes like in HN comments as well?
I have tried it and found nothing useful for me, but am interested to know why/how people use it.
You guessed it right. No, it is not my first language that's why maybe I am more inclined to use paid tools like Grammarly. Although, my coworkers/employees are native speakers they make more spelling mistakes than non-native speakers (maybe confidence?). I force them to use grammarly and almost every email there is 5-10 corrections. If you didn't try yet, just download and scan some of your old emails. I bet you will find minimum of 5 mistakes. Better safe than sorry.
Are we cool with them now or are they still stealing your data? I'm not up to date