I have been using Grammarly for a couple of years now. For the most part, I have found it useful. I used it the most as the Chrome extension when I was line editing short articles for my local school district’s e-newsletter and social media. It helped me to catch many, many mistakes I likely would’ve otherwise missed. I also used the Word add-on to do the last check of my books. This helped me to produce a totally clean manuscript (I believe).
I also have used the Outlook Add-in, but find myself disabling it quite a bit because it slows down the program. I haven’t used the feature where you upload your file to Grammarly.
I had the premium version of Grammarly which cost me $6.56 per month billed annually. As a premium user, I not only got grammar errors flagged with a red dot with the number of errors (or a green “refresh” icon if everything is fine), but I also got notification of lesser errors, denoted by a small yellow dot. When I expanded the menu and clicked to see the “yellow dot” errors, I almost never (if ever) chose to accept those suggestions. Though I agreed those suggestions would’ve made my writing technically immaculate, it seems like they would also take my voice and interest out of my writing.
In summary, I recommend the Chrome Extension and Word Add-in but would disable the Outlook Add-in except on the most important emails just because of how it slows Outlook. I will also consider dropping my premium subscription when my period ends because I simply don’t use the expanded grammar suggestions.