I taught economics to a group of third year students at Simon Fraser University this spring. Perhaps 80 percent of them spoke English as a second language (most were from China), and very few of them had experience writing in English, as the Economics department put most weight on math skills.
I went against that trend in asking students to read and write about Economics in two briefings (here's the PDF for the first) and a blog post (remember these?).
I had given the same assignments to my 2009 UC Berkeley class (blog archive), but I had not thought about plagiarism as an issue because I had skimmed through most of their briefings (I failed one student for a ridiculous copy/paste from a sociology paper).
I was going to do the same this time and asked students to print three copies of their briefings so we could distribute them to other students for peer grading (see this post). Then it occurred to us that we could use online submission to make the process easier (and maybe save a few trees). That step gave us a new possibility of checking the briefings for plagiarism. (We were not interested in grading or correcting them, as that was for peers.)
By curious coincidence (not!), I got an offer to try out the grammarly service, which helps students and professors check writing for grammar and plagiarism issues.*
In my short use of the service, I did not pay much attention to the grammar function, although I can see how it would be better than Word, which tends to vary between terse and useless.**
I did spend a lot of time with the plagiarism function and found it to be VERY useful, especially as nearly 40 percent of my students plagiarized material for their first briefing. Most of them made the mistake of copy/pasting without "marking others words," but some were brazen enough to copy entire paragraphs without attribution.
Most of them lost half their points on that assignment (their midterm and final grades tended to be lower, so I think there was some self-selection), and few of them did it again on later briefings and blog posts. (I was really shocked when some did!)
Bottom Line: Grammarly makes it easy to check lots of material for obscure sources on the internet. I recommend it to anyone who needs to grade a lot of essays for original content. It's also handy for grammar feedback (you can print a PDF report), but I wouldn't assign grades for grammar.
* Grammarly offered to pay me to review their service (this post), but they've been totally hands off on what I say. They did implement one of my suggestions on how to improve the interface. The service costs $30/month, but many of my students enrolled in the 7-day free trial to double check their work after a bunch got caught plagiarizing (see above).
** My grammar and spelling is terrible, so I don't mind some highlighting. I find more mistakes by rereading my work, but some people cannot do that at an "objective distance."