I was initially processing out 1920x1080 ProRes files from Cine-X and then working in FCP. Every time I wanted to adjust a file I would go back to Cine-X, make my adjustment, re-export, re-import in FCP and replace the original clip (PITA). PremierPro, however, can work with the R3D's natively, even apply the adjustments available in Cine-X or reload data from RMD files if you rework the originals in Cine-X.
I think I'm about to swap Apps as I'm going to be working on a short with a filmmaker friend and want to maintain a 4k workflow, which is a pain in FCP. Plus the fact that CS5 PP can access my mountain of RAM!
Thankfully I have a subscription to Lynda.Com and PP isn't all that different from FCP.