Amazon Mechanical Turk is a very odd system for working with writing subcontractors.
It doesn't allow the freedom of communication that connecting with writers on web forums does. It doesn't have the full suite of process management features that RentACoder does.
What is does offer is speed. If you put an article request on Amazon Mechanical Turk, you can get a completed article back in an hour.
If you're requirements aren't very specific and your time frames are short, this might be something you would like to try.