Only installed it on Sunday, but I can absolutely confirm we love it !!! Bear in mind we have upgraded from a 7yr old (or thereabouts) Panasonic Viera, so anything would have shown a noticeable difference.
Due to personal circs at the minute (elderly care), I haven't had time to 'bugger about' with it ..... but I have found it very easy to get to grips with. As with all things new, it's the navigation of the device that is time-consuming, but after a couple of days the basics are soon picked up.
I have viewed a couple of freebie 4K/UHD vids and when this tech really takes off with the masses, it is easy to see why it would. It's like 'you're there' it's that clear/sharp.
Regarding size (oo-er). We have chosen the best set based upon what all the research suggests and it is absolutely fine. Any bigger and we would be having to look 'around' the screen, eyes dotting about all over, to see what was going on. With the sizing guide you see the whole screen in one go, if that makes sense ?? Bigger does not mean better in such circumstances.
My only regret is not buying a sound-bar at the same time. Whilst visiting my Mum she showed me the difference in sound with the bar off, and then on ..... again, it is very noticeable therefor a 'must have' in my eyes. Don't get me wrong, the in-built sound from the TV itself is adequate enough, just working on a cost-effective basis I think circa £150 for a sound-bar will benefit us.