I have a 55" 4K display on my desk. I game on it all the time.
Me too and if it takes .1 sec longer to fire up i cringe, why, I couldn't go back to my old 32" and it would be a bit pricey to replace. Mine is a curved Samsung TV. There are a coupe of downsides for sure. While I no longer play competitively (actually did at one time) FPS, this screen does have some tears in FPS, i never notice when Im playing but if I watch somebody else I can see them, you need to want to see them though, almost un- noticeable if you are not looking for it. Also the screen is not as side view friendly as my monitors where. As a spectator looking from a side stuff on the extreme left (if you are on the right) can be a bit blurry. Not for me in the center, just spectators. SO Hardcore video people need to wait. The biggest downside and some have mentioned it here, is the grunt it takes to power it. I was on a 980GTX and went to the 1080 gtx, and still find a game or two that cant pull it off full settings, not many (i consider 30+ acceptable, some of you wont, most my games (almost all) are closer to 60+) I think one more generation of Video card should put us in 4k for all area.. which brings us to the worst thing.. it costs a boatload.. actual screen is $700-1200, dependent on what you buy, MIne was $900 on black Friday (still lists for about $1200 today, but its a generation back), $600 video card, and you do need a current PC ($1000+ if you build, less if its upgrading).
In the end I could not go back to the small screen, games looked NEW again to me.. now the NEW has wore off, but I wouldn't want to switch back.. Even for work type stuff its a huge change. you can get smaller 4K monitors but throw up 4 excel,word, Chrome screens and read them easy (i'm old) compared to a smaller 4k. I use mine for 90% gaming though, and have never once felt it was a bad purchase. As prices come down i may change that opinion, only as better screens will be cheaper then mine though, not that i regret the size change.