I've been running 4k for a few months now, on a LG 27UD68P monitor. My previous monitor was the same size, 27", but it was a Korean import Achieva Shimian 2560x1440 model.
I was running a 980Ti until recently, and although on a 27" 4k monitor AA is completely unnecessary and results in decent framerates with that GPU, I do miss getting near constant 60fps+ with 2560x1440 with it. Ended up buying a 1080Ti just so it bothers me a lot less.
One thing folk don't talk about much when discussing 4k gaming is just how badly some games scale to high DPI panels. Mass Effect 1 and 2 are great, they scale independent of resolution... and then suddenly ME3 has fixed assets that are miniscule on 4k without mods. And don't get me started on games like Deus Ex: Human Revolution - the text is about half a millimeter real size on screen, it's awful, and there's no way to fix it. Console ports do not always do well running at native 4K, and it doesn't really scale to a resolution other than 1080p exactly, which is a shame.