And now from the other side of the coin:
I've used a 4K TV as a monitor since the Seiki 50" 4K was released. I have been very pleased with the increase in screen real estate. One thing to keep in mind is that while even the intel iris 6100 integrated graphics will push a 4K display (I'm typing this on my work machine with a Core i3-4160 using a Samsung 4K display via Display Port) for gaming performance you will need some hefty power to get "great" framerates. (I use a pair of pascal-based Titan X cards at home)
From a productivity standpoint, I can open excel and see 54 columns and 93 rows which allows me to work with complex spreadsheets and see "the whole picture". Or, I can have email open in a 1080 window, powershell window in 1080, Explorer in a 1080 window, and a browser in 1080 simultaneously. Can't find anything to complain about with that at all.
For me, it was worth the upgrade and I never looked back. You have to determine if it's beneficial to you though...