The Retina iMac has only just enough graphical grunt to push the "5K" display, even in its tricked-out configuration. It's always worth remembering that the Mac Pro is the only computer Apple makes that isn't based on laptop parts.
Whilst I have no doubt whatsoever that the 5K RiMac will look positively edible and have a screen that makes grown men swoon, I have my doubts about its long-term reliability. Buy the AppleCare! More worrisome is that OS X is not, apparently, on the same frugal trajectory as Windows, so future revisions of the OS will need more resource, not less.
Whilst I have no doubt whatsoever that the 5K RiMac will look positively edible and have a screen that makes grown men swoon, I have my doubts about its long-term reliability. Buy the AppleCare! More worrisome is that OS X is not, apparently, on the same frugal trajectory as Windows, so future revisions of the OS will need more resource, not less.