I got the SEIKI 4K TV from TigerDirect not long ago. I hooked it up as a 4th (!) monitor. It dwarfs the 3 30" dells I have next to it since, well... it's frikin 50"!
Despite being a lot bigger the pixel density is roughly the same as the 30" Dells which are only 2560x1600. The SEIKI 4K is rocking, obviously the 4K resolution of 3840x2160.
So is it cool?
The fundamental problem, of course, is that the refresh rate is only 30 hertz. This is driven by the fact that current 1.4 HDMI spec can't push faster than that. So the screen has a soft pulsing. It also tears badly on fast moving things, but this may be a separate issue not related to the TV, not sure. Been messing with my video card to try and solve that. VSync doesn't seem to help, so maybe it is the TV.
Color reproduction is just ... meh. You have to switch modes to get things to look right depending on what you are doing... say work vs. play. Games do look spectacular at the high resolution and the big size. I have the monitor at a normal seated distance, so it's ... immersive. Much like the Rift in that way, but without the nausea and fatbits.
The bottom line is, don't get this TV unless you are a crazy early adopter who just likes cool toys and throws money away to do it. Wait until next year when HDMI 2.0 comes out and more monitor-class 4K units come onto the market. Then, yes... if you are a resolution junkie like I am, get one! Because even in this early form, the promise is quite clear.
Oh, and it impresses friends. Very important point. :)