Grammarly is fine if you already know enough about grammar and mechanics to know when it is steering you right and when it's not. Unfortunately no grammar checker is perfect (though anything would be better than the sad excuse for a grammar checker in MS Word, I think!), so for best results I would only use it to catch the worst errors in a first draft, then send it off to a human beta-reader with good writing skills to correct its corrections.
Also, IIRC Grammarly has a charge for use (although maybe it has a free version as well?), so it may or may not be worth paying whatever they're asking per month, depending on how much you write and how much help your writing needs. Everyone's mileage will vary on that question, of course.