For me (and not just for me, quite a few of my colleagues had the same problem) 2560xsomething is too much on 27" - the letters are too tiny, your eyes hurt after a longer work, and subsequently your head hurts. (I’m talking about office / development tools, I assume it’s OK for watching movies ;)). Sure, you can set bigger fonts, but then some programs may start to look weird, when you connect to your machine via Remote Desktop, the letters are too big or tiny (don’t remember exactly which one)… I’d rather stick with ordinary 100% magnification.

I have 2560x1600 on 30", that’s fine - but for 27" I prefer 1920xsomething (I bought one 27" and I specifically went for those having 1920x, not 2560x).