4K monitor? Sure. I'd love one. 4K TV? No thanks.

I feel the same way as Anne, I can't wait for a higher rez 27" display. 1080p for on a monitor larger than 24" just doesn't look good. When sitting this close to a display it needs to be higher resolution.
I will pick up a 4K TV eventually, when my 1080p TV dies and 4K is all I can get. Not before that.

Saw a 4K TV at the local Mart. Expensive but absolutely amazing. Can you see that difference to a 1920? Sure can
Over the air content has to catch up that's for sure, but these days people also use their TVs to plug into computers and consoles. And display port 1.2 can output it just fine (which most laptops and video card have)
Plus what most people don't know is that video post processing power has also scaled with new TVs. I saw a 1920 movie scaled to 4k on a Sony, and unless you paused the video you would think it was native. Sony of course pulled the same trick for scaling 720p up tot 1920. Something to keep in mind.
Glad they are taking 4k seriously because the jump will really be like VHF to Bluray

Saw a 4K at the local shop and it looked amazing for panoramic shots but the movement of even small boats in the background was totally pixellated and looked truly horrendous. It reminded me of the first HD screens that were rubbish for sports as they couldn't keep up, only 100 times worse. The problem probably is the difference in quality is now more pronounced rather than necessarily worse. I am guessing that for sports or anything with significant movement , waiting is definitely the best policy , though I have only seen one so far so I could be wrong.

dpp1.2 isn't a good solution for 4k monitors, to have 60Hz at 4k you need to split the monitor in 2 virtual displays. i'd rather have a monitor UHD>res>FHD with 16:10 and 144 or 120Hz

So... you can buy a $100,000 Mercedes AMG coupe for the same price as a $25,000 Chevy Cruze. It is unlikely that you will ever be driving on professional Race Tracks, and pushing the limits of the AMG; but just the idea of power and control and the option makes this deal a no brainer.
It should also be obvious that the manufacturing costs of 4K are rapidly approaching the levels of HD. Even if you claim that you cannot see or need the 400% quality improvement, no manufacturer in the 21st Century is going to continue to make and market lower quality products that cost the same to produce.
Consumer tech is surfing its own less extreme variation of Moore's Law. The advancements are slower, but the rush is no less exhilarating; and only the recalcitrant Luddites will want the better for less to go away.

" given that the human eye isn't likely to be able to tell the difference between 4K and 2K in most viewing environments?"
1) Unless it's a placebo effect, and my eyesight is not even that good, I was amazed at how 4k resolution looked, compared to blu ray. 2) Was at the Sony center in NYC and the rep said the current TVs they had on display did not upscale to 4K, but only up to 2K, so forget about watching regular HD cable (1080i where I live) as 4K.

Actually the analogy of number of pixels with cameras is not totally valid. With cameras higher pixel resolution allows cropping of the image or zooming in depending on what you are trying to do and still end up with a usable resolution (provided the lens is good enough etc. For viewing then the argument becomes less convincing, as the eye does have limitations but the again I remember the argument that you didn't need HD resolution on a 24" TV - you do!

" given that the human eye isn't likely to be able to tell the difference between 4K and 2K in most viewing environments?"
I don't know why this rubbish seems to continue to be pushed around. It is absolutely possible to tell the difference between 4k and 2k in most viewing environments. The only people who say this are people who have bad eyesight or who have never seen a 4K tv. It is EASY to notice the difference between 4k and 2k and everyone I know that has seen one agrees that it is significantly noticeable.

I saw a couple of 4K 80 inch screens and I sure could tell the difference. In fact I was amazed because it was so much better than the other HD tvs on display. When you say that people can't tell the difference then I've got to think that they need to clean their glasses or get a new prescription.