I got my 27" iMac in 2011. In hindsight, I might have preferred a 21" iMac and pair it with another 21" monitor instead.
Here are my experiences:
1) For gaming, the 512mb graphics card cannot power many games at native 2k resolution at decent performance. I ended up running most games in 1080p windowed mode in bootcamp.
2) The font is actually a bit too small for my liking on a 2k, 27" monitor. I end up having to zoom in to 200% on most documents and enlarge websites, which kinda defeats the purpose of a larger display, and it's a bit irritating that Safari cannot remember zoomed settings the way Chrome does. Conversely, 1080p on a 21.5" monitor is just right for me.
On a side note, I love the new font sizes on the 5k iMac, and I find it "just right" for me, but it's too pricey for me. That and my current iMac is still going strong 4 years later. Which is more than can be said for the barely 2-year old HP desktop that I ended up getting the iMac to replace.
3) I actually had eye strain for the first few weeks after getting my iMac due to the small system fonts. I eventually got used to it. That said, I probably won't experience this issue with the retina 5k iMac.
4) I find I didn't really need the extra vertical screen space, but I sure would have appreciated more horizontal screen space (since I work heavily with documents). I often have a lot of overlapping windows because there isn't enough space to align my apps side-by-side.
5) I don't have to work with apps like logic cut pro or photoshop, so I guess I don't really "need" the extra screen estate.
Problem is - the 21.5" iMac feels kinda gimped (entry level uses integrated graphics, and you can't upgrade the ram yourself), and Apple seems to be pushing everyone towards the 5k iMac option, and I really don't like the idea of running a 5k display with hot graphic cards under such a thermally-constrained environment. I will likely hang on to my current iMac for as long as I can, because Apple's current lineup really doesn't entice me in the least bit.