I guess it's useful if you're terrible at writing, and it's helped me fix a couple minor errors, but overall this tool is terrible. It recommends putting commas everywhere you could possibly think of, misses a ton of grammatical errors, and is generally not useful. The thing that drove me over the edge was when it would recognize "Dursley" and "Dursley's", but would correct "Dursleys" back to "Dursley's".
It's more of a nuisance than a help.
