I never use these kinds of grammar-checking, writing-analyzing tools. Except when I occasionally want to amuse myself, since the way I like to write tends to make these things go nuts.
The Hemingway Editor is not so much a grammar checker as it is a sentence structure analyzer, and a dang good communications simplifier—it seems to absolutely hate pretentious, “trying to look smart” writing. It will go to town on your long complex sentences, big words, passive voice, adverbs, and just about anything else that takes away from simple, clear, and direct. And that’s a good thing.
Great for business communications and blogging, I’d say. Although creative writers will likely give Hemingway an aneurysm, it’s good to check in every now and then to see what Hemingway thinks of your work.
What I do find particularly valuable is Hemingway gives your writing a grade level Readability score. And a few whacks across your knuckles with its ruler for good measure, since it knows you’re “always up to no good anyway.”