Sam Y.

I'm a professional editor and writer (NY Times, Psychology Today,
Men's Health et al.), and the author of a style and usage manual.

Grammarly is crap. I signed up for a free trial (and damn, it was hard to remove the
Thing from my computer afterward), and submitted a short paragraph simply
Dripping with errors of spelling, grammar, usage and punctuation. Grammarly
Missed every one of the mistakes; it flagged only one word as a possible
Error---one that was not an error at all.
I distrust all spell-checkers and other computerized editing tools, so yes, I was
Biased going in, but Grammarly's performance in my brief test more than
Justified my bias. Stay away from this program.