I have Lenovo Y50 4K and use Seiki 50-inch TV 4k TV which I sometimes use it as a monitor for my machine. 4K resolution is amazing but there is definitely diminishing returns. I am sure 5K would be definitely amazing but do not expect a lot if you are power user or a programmer. Designer and other artistic folks may have more to mine here.
The 5K is a result of the Mac having an aspect ratio other than 4K & HD's 16:9, and resolving that difference with using a simple "retina" multiplier. Kicking 4K's butt when it's barely out the gate probably helped.