If you can see and are bothered by the individual pixels on your current TV, then no, they are not a waste of time. Waste of money though, maybe...
However, most people I know personally who have bought them are just doing it because 4K is 'better' and that's the beginning and end of their opinion on it. For those people... yes, it's a waste of time, but at least it makes them feel better?
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My opinion is that with the viewing distances and TV sizes that the vast majority of people use, 4K TVs are going to waste.
For computer monitors though, since you are typically much closer to them than you are to your TV, 4K can make a noticeable difference. But the quality differential isn't so large as to make going back to 1080p painful. My every experience with real-world 4K (as in non-tech-demo) has been 'yeah, that looks a bit better' followed by wondering what the big deal is.
The drop-off between SD and HD was massive and immediately obvious to everyone (except for a few people who I'm pretty sure are lying about not being able to see the difference). HD and 4K shows some severe diminishing returns. And I fully expect the difference between 4K and 8K to be imperceptible.
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You specifically asked about TVs, but I will say that 4K is probably going to be relevant for VR.
And I will also say that 4K screens are really stupid on phones. I'm looking at you Sony.