I owned a SAMSUNG UN55HU7200FXZA - (55" curved screen 4k), for about two weeks. We wanted to add a 2nd TV in our large living room/family room.
Although the 4k was nice and the picture was nice, we ended up preferring our old 09 Samsung 50" plasma (PN50B550). Why? Realism of colors and wide viewing angle.
Also, the 4k content was nice, but we played some YouTube 4k content in parallel (fantastic Costa Rica footage) and unless one was sitting within about 4' or 5' from the 7200, both TVs looked very good, and comparably crisp and clear. I know the new 4k Sammy wasn't calibrated but then my old 50" plasma has never been calibrated either. In fact, there was a warmth or realism to our old plasma that we favored over the new 4k 7200. Don't get me wrong - the 4k 7200 at a distance of 4' looked very very crisp and pristine!
So, I took back the 7200 and went out and bought the 51" 8500 and am totally happy. In fact, I plan to add the 64" 8500 to our basement before the end of the year.
I figure that 4k OLEDs will be in my future perhaps 2 or 3 years down the road. The next TV to go would be our old Sammy PN50B550, so that will be a good excuse to get an OLED in a year or two!