I sat in on a panel they hosted at the Austin Film Festival - back in October. At that point, I think they said they had something like ten thousand submissions. And had optioned twenty two of them. They had been up and running for over a year.
You can download their contract terms and read all about it. It's certainly stacked in their favor - but they are for 'real'.
You can submit 'publicly' or 'privately'. Your call. The easiest thing is to go to their website, read the FAQ and download their agreements.
Someone asked "Who reads the submissions? College kids?" - The CEO smiled and said, "We're Amazon. We have a development deal with Warners. We can pay people to read the submissions."
Just thought about the numbers. If they got ten thousand scripts in close to fifteen months. Say they're getting a thousand scripts a month. Something like 35 submissions a day. If they hired five FULL TIME readers. (They're AMAZON for goodness sake) Then those readers would have to cull through maybe seven or eight submissions a day. And I've seen a screenplay slush pile - you can throw out a LOT of scripts in the first ten pages. So it's not an unreasonable work load.