Just got the 50" for $449 before tax on Amazon. Best value for the dollar ever. Beats the crap out of my Insignia 42" 1080p set.
Had some oddities when setting up, so I'll go into it here in case it may help someone.
1080p has overscan below 120hz. I'm now using the Custom Resolution Utility, and am switching between 1080p 120hz and 4k 30hz through the Nvidia control panel (also made custom resolution here).
I had flickering problems, sometimes frequent, sometimes stable. I had to power cycle the TV and restart the computer to get it relatively stable, but switching to a generic HMDI cable fixed the flickering for good. Before switching the cable, the flickering seemed to start when I changed resolutions or displayed fullscreen video. Firmware update did nothing for the flickering.
Runs butter-smooth in 1080p 120hz. 4K's motion is a bit weird, but I haven't tried 24hz for my movies yet (ripped BD, not an external player). Games look fantastic in 4K, but that motion is too much for normal use, at least until I get another GTX 760 to run in SLI.
Final thoughts: After some calibration using test patterns, and switching to a different HDMI cable, this thing is a BEAST and gives me no issues. Looks wonderful and smooth and big. Super happy customer right here (especially since it fell over after I received it).
i7-4770k, GTX 760