Been using Grammarly for about a year without an account and have found it OK. I say OK because it works nice, but not any better than the built-in spell checker in Firefox.
Grammarly is slower and would often appear to be working but wasn’t. This meant I had to double spell check posts online.
The latest update which ‘I think’ landed around 14th Dec doesn’t seem to spell check, and a new pop-up comes back on every page upload. Guys, this isn’t going to help me sign-up to your service!