Something to keep in mind is that this is effectively a Windows monitor, not a Mac one. The reason for this is PPI: Most Apple displays are in the 100-110 PPI range, with Retina Display Macs doubling that to 220 PPI. At 28", a 3840 x 2160 panel has a PPI of 157, which sits right between Retina and non-Retina densities. This means that on a Mac, you’ll have to use it one of two ways: Either at 1x, where the higher PPI…
Huh. Are you maybe thinking of iOS, or have you confirmed this with a high-PPI external monitor on a Mac? Because it's not true of the built-in screen for retina MBPs. I typically use the 1.5x resolution (1920x1200) or 12/7x resolution (1680x1050), which are both supported by the built-in system prefs, and both look perfectly sharp as far as I can tell. Other resolutions are available via 3rd-party hacks and seem to work fine.
The sharpness is supposedly because things are rendered at double resolution and then scaled down, which required some clever GPU work; whatever it is, it works for me. I'm prepared to believe that things don't work as well for third-party external monitors, though. Or maybe we're not ready yet to handle 60% more pixels, when the rMBP is already pushing performance limits?