Confession: I am an English teacher, and I use Grammarly.
I first installed Grammarly (the free version) to test it and decide whether I could recommend it for my students. (See my analysis here.) Overall, research shows that Grammarly has a positive impact on language learning, so I recommend my students use it.
I've found that even as an English teacher, the tool is very useful in quickly finding errors. On this website, when I open an old page, I can now quickly find the errors I didn't see when I wasn't using the Grammarly plug-in. Here is an example:
In other words, even if you are a native English speaker, it's great for spellchecking and proofreading. You'd be surprised how many lazy errors or mistakes you make even if you know the rules of grammar.
Grammarly Drawbacks?
I've noticed in some form fields (boxes which you type content in on a webpage), the plug-in operates a little slowly. Similarly, in my Gmail compose window, the plug-in slightly slows down how quickly the text I type appears on the screen. If you notice this, the easy solution is to disable Grammarly for particular sites. (Click on the icon on your toolbar, and un-check "Check for writing suggestions on mail.google.com'.)
When I notice some lag (slowness), I disable it. In the end, though, I'd rather have it enabled (especially for important e-mails) to find the silly mistakes I make.
Conclusion
I had only planned to use Grammarly for a short while to test it, but I've kept it on since. Even if you're an English teacher, the plug-in is useful for proofreading. I use the free version that can be downloaded here.
- Matthew Barton / Creator of Englishcurrent.com