(I wish to give a giant disclaimer: I do not think this is a good resource for grading. I do not use it as such. I would never use it as such.)
Now that we all have gotten over the shock that a computer can grade, maybe take a step back and try to figure out how this excellent program can be used in a positive way.
I have yet to try it in class, but for the next essay, I plan to incorporate this with my peer review day. It is a free service so I had to try it out. I have digital files of my students' essays. The site is simple enough. Simply cut and paste the essay into the box provided. Then it asks more questions, such as type of writing and grade level. You can even submit your Works Cited. After you submit your writing, it produces some very intriguing comments ranging from spelling and basic grammar to use of vocabulary, transitional words, etc. It even counts the number of pronouns, which I find useful since students can easily bog down a paper with pronouns.
In the end, it gives a "grade" with a disclaimer that in no way this means the essay should get this specific grade. I tried it with a few of my essays and the grade was "in the ballpark."
Again, I do not think an instructor should use this to grade. Repeat, do not use this to grade, but I think students could find this site extremely helpful. Another set of eyes (even though it is a robot) is beneficial for writing. So give it a try and let me know what you think.