Frederick L.

Okay, I'm talking about the paid, premium version here. In my case, I used the Microsoft Word add-on.
In its favor, it does actually catch a lot of errors that Word does not. That's about the only good thing I can say about it.
The most basic of features are absent. I can't take a passage and tell it "Don't review this, it's in a foreign language."
The other downside is that it stores errors on a server (I think!) and not in the document itself. This requires a constant internet connection for it to function. Yet, even though I have a very good internet connection, it still has problems.
For example, I'll go through a hundred errors in a document and click "ignore" or I'll accept the correction. I'll save the document and close it. When I open it up the next day, Grammarly will have forgotten all of the edits I made and I'm back to square one.
Also, Grammarly will tag parts of words that are misspelled. E.g. If I have the word "would", it will highlight "woul" and say it is a misspelled word. I hit "Ignore" and the next time the document opens, the "error" is still there.