I thought that this was a great grammar tool, but when I Started watching a video, The video would just not appear. And when I removed Grammarly, The video worked absolutely fine. I was not signed in, so My guess is that If you don't sign up/in, they will break the website you are on.

When it works it works very well. About one half the time it signs it's self out. Doesn't pick up mistakes on Facebook. Adds commas where they don't belong.