I've been using the paid version of Grammarly for the last 4 years. It's been working very well and have helped me catch some errors for sure... as well as improve my writing on the whole.
However, I've been having issues with its performance (technical) for the last 6 months or so. My computer's CPU would be clogged and working at 150% after a few minutes of having Grammarly open. It feels like they are using my browser to mine bitcoin or something. If I forget closing the tab with Grammarly, my laptop's batter will be drained in half an hour, and it will be ready to fly to the moon. I can't tell what they are doing exactly, but it's definitely abusing the access to your browser and the CPU.
Apart from that, it works as good as one can imagine.