I've also been the one on the opposite side of the classroom. I was lab based, so we didn't use Turn it in.
With a reasonably sized class, you can easily spot which students have worked together because their reports tend to be shockingly similar.
I agree that you get a feel for them with informal conversations and you can see how their submissions tie up with your informal conversations.
I used to tweak the questions year on year. I've suspected there is a black market, an assignment exchange, or something because I caught students submitting work from previous years. They were mainly international students that were only there for their masters year.