So, I was originally shopping for a low priced 4K TV for my home office in the 40"-50" range, I never experienced 4K at home, I have 1080p 63" Samsung Plasma and 1080p 3D BenQ Projector and 105" screen for my home Theater needs.
Considering how cheap 4K TVs are right now, I decided to get one just to "play" with it for 4K TV/video viewing, then I found the 40" 4K HDR(sort of) Samsung KU6290 for $289 at BestBuy for this holdiay..which according to rtings
http://www.rtings.com/tv/reviews/samsung/ku6300, it is the same as KU6300 but without Bluetooth and smart remote which I couldn't care less, and it is actually GREAT as PC monitor because it supports 4K@60Hz and Chroma 4:4:4 with low input lag in game mode of no more than 36ms.
I got it and decided it to try it first as my monitor instead as a TV for my office, going from my Acer 1440p 144Hz TFT monitor, I didn't know what I was expecting...but after an hour of using it...I was blown away
Dear god, 4K on 40" is like having a personal IMAX, my eyes are still getting adjusted to the sheer size of the screen, but I love it
Only issues is screen tearing with vsync off specially coming from 144Hz, and 4K gaming performance on GTX 1080, the 1080 is enough for almost all games (except Crysis 3) at 4K with maximum settings, but certainly would be looking to get GTX 1080 Ti or Vega
Screen tearing is annoying, I turned on vsnyc in a lot of games, and honestly I don't feel much input lag, specially if the game supports adaptive vsync such as Doom, so I am OK with it.
So, even with some screen tearing, and getting adjusted to the lower frame rate for now, it is a completely different experience to have 40" 4K monitor 2 feet from you..I am only concerned about my eyes though

..still.
So here you have it, I am gaming on 4K now...and love it.
PS: I don't think it is worth it to go 4K as a monitor on screen less than 40", just not worth it IMO.