The Dell UP2715K 27-inch UltraSharp 27 Ultra HD 5K LED Backlit IPS Monitor works on the 2015 MacBook Pro Retina: simply plug in the supplied double Mini DisplayPort cable, set the display to 5K mode in the menu, start up the Mac and you’re done.
More coverage coming on the Dell UP2715K 27-inch 5K display.
Main comment: it is a shiny display like the iMac 5K; as with the iMac 5K, the first thing I saw on the screen was my backyard (reflection). Other than that, the display is hyper sharp gorgeous and just like an iMac 5K.
As I wrote in my review of the Apple iMac 5K, getting an iMac 5K is like buying a nice Mac with a free 5K display—something to ponder since the base pricing of the iMac 5K (about $1899) is similar to the Dell UP2715K display alone.
Uses two Thunderbolt cable to drive at 5K
The main downside is that (on the MacBook Pro) both Thunderbolt ports are taken up in order to drive it at 5K, leaving no Thunderbolt ports on a MacBook Pro.
This is soluble: connect the other Thunderbolt devices first, then connect the display to the device Thunderbolt port (thus a few devices with only one TB port cannot be used), aka daisy-chain them.