I know there's a lot of hatin' on Turnitin going on out there in the Composphere, and I am sympathetic. I really am. I had my students submit their research papers to Turnitin, though, and I think I'm glad I did. They submitted rough drafts, not final drafts. I wasn't trying to do a "gotcha" thing. I just wanted to have an object lesson in paraphrasing without subjecting myself to reading 3000 (bad) paraphrases.
It's worked out pretty well. They bring me their reports from Turnitin, and I tell them which parts really should be reworded and which parts strike me as insanely silly. It marks things like "in the United States," for example, as plagiarized.
I have noticed that no one yet has submitted a paper Turnitin said was 100% plagiarized. My theory is that the people who were planning to cheat have opted instead to just not turn anything in.
So if I have a point it's that automated plagiarism catchers can be made into useful exercises if the power to decide to take their advice or not is in the hands of the students. That is all I have to say.