Personally, I feel it's good just to see what errors you might've made: grammar, spelling, or otherwise. It's free so you can just try it out and see if you like it or not. I would not suggest paying for the premium version unless you really want to be a professional.
all these editting programs (pro writing aid and autocrit are the other two main ones) are okay but prone to errors, i wouldnt recommend them for a non native speaker who might not spot when the program is wrong
If you use Chrome, it has it's own spell checker , you don't need it for typing on screen . I use word , so don't need it there . I have used Grammarly , and it works OK. But, I found it became a bit intrusive and the constant silly Emails got on my nerves . I have been trying to improve my grammar , it is a struggle , but you need to become more analytical of your writing . Spell checkers can help you to become lazy .
I think it's a good resource if you will know when not to take it's advice. It is right more often than not, but not 100%. I have not used the premium/paid version so can't speak of it.
Like everyone else has said, it has its share of errors. It will correct something to the conventional form, and it won't read what you wrote in the same way you do. I personally use it for grammar errors, like misplaced commas and that sort of thing.
My recommendation is that if you have doubts about grammar, use English learning forums to correct your mistakes.
