I have the Seiki 50" version of their 4K monitor. The 39 inch version shares the same limitations and benefits.
The quality of the picture produced by the monitor is all that I can ask for. Having 4K of usable desktop space at home makes me hate my tiny little 1440's at work. The best part of having 2160 vertical space is the sheer amount of code that I can see in each IDE. For some reason, tilting a normal monitor to stand in portrait mode bugs me. Too little horizontal space engenders its own type of claustrophobia, I guess.
The only problem with the monitor is the poor refresh rate at 4K resolutions. I can tease 120Hz at 1080p, so its great for gaming, but at 4K I am limited to 30Hz. The 30Hz refresh rate will either result in signifigant input delay, with desktop vsync enabled, or, with vsync disabled, will result in lots of tearing every time you update a large portion of the screen (scroll the screen or move a window, etc). I can't recommend the 30Hz versions of 4K monitors, unless you know what it's like.
I'd recommend that you turn your current display into a 30Hz display for a few days, see if you can stomach it, before buying a Seiki.