I bought a 65" 4K TV around 8 or so months ago. I had recently moved from an apartment to a house and the viewing distance to my current TV increased a fair amount so it seemed too small
There was a one day special on at Best Buy when I bought the TV, so I justified it to myself because of the big saving (the same set is still going for nearly $1000 more than I paid for it).
Most Netflix 4K streams don't look hugely better than 1080p ones other than the pointless showcase ones that are just shots of rivers and mountains etc. Some 4K YouTube videos look great, as does stuff I record in 4K on my phone, but overall I would say that if I'd bought a new TV specifically for 4K then I would be disappointed. Obviously the only way to get top quality 4K content will be to buy a UHD Bluray player but so far there aren't enough movies available and the discs are still extortionate.
I traded my PS4 for a Pro to get more 4K content and I would say that I'm fairly pleased with the results. HDR makes more of a noticeable difference to me than the resolution though, Ratchet and Clank is probably the best looking game I've ever seen. (I don't have a very good PC).