A couple of years ago, because nobody else was doing it, I undertook a comprehensive, comparative study of 20twenty text-checking programs, including Word’s, WordPerfect’s, Grammarly, WhiteSmoke, StyleWriter, and others. I used test documents I found in books or on the Internet, not ones I made up. The detailed results of these tests are published at Grammar Checker Comparison Tests. They show Grammarly as a middle-of-the-pack program, catching about 20% of the 350+ grammar mistakes, and about 11% of the 350+ stylistic problems in the test documents. I’m sure some of the checkers have improved modestly since my study. Grammarly has certainly spent lots of money in advertising, and is probably the most widely known.