David Crotty has published "Thumbs Down for the Freemium Model? Researchers Reject Nature's Fast Track Peer Review Experiment" in The Scholarly Kitchen.
Here's an excerpt:
NPG launched a four-week trial in their megajournal Scientific Reports. The journal features a Gold open access (OA) business model, where accepted authors pay a $1,495 article processing charge (APC). In the trial, authors willing to pay an additional $750 upfront would get their decision in three weeks. NPG would be able to offer this additional speed by outsourcing the peer review process to Rubriq, a service offered by the private company Research Square.