Mike Burnhill is with CPS for Canon Europe and did an interview with Fotosidan about the EOS 5Ds, EOS 5Ds R and how the product came to be, and what the design philosophies were for the camera.
Mike touches on why a large megapixel sensor isn't ideal for 4K, or even 6K video, which sounds like we'll see that addressed in an upcoming full frame EOS camera. It's also interesting to note that Canon L lens design since 2010's Canon EF 70-200 f/2.8L IS II has had high resolution image sensors in mind, and that every L lens since has been designed for use with these cameras.
You'll get a pretty simple answer about whether or not shooting with the “R” version of the camera is for you. As well as why Canon decided to stick with Compact Flash over new technology such as CFast 2.0.