Just got the Dell 24" 4k display today.
Wow, the double resolution is so so nice. My laptop can only drive it at 30 Hz, but for everything I want to do, it’s still really dramatically better. Looking at photos, text, maps, etc., everything is super super crisp. At a typical viewing distance of ~2–3 feet I can only barely resolve the individual pixels (whereas on a 24" ~1020p display, the individual pixels are very obvious). Hopefully within the next year sometime I can get a desktop computer that will drive this thing at 60 Hz.
It’s much nicer for e.g. reading PDFs on screen, or editing photographs, or examining fine details of diagrams. But it’s also much nicer for just browsing the web, writing code, drafting emails. After I get used to this, I think it would be pretty hard to go back.
~184 ppi doesn’t sound that amazing compared to modern smartphones, or even laptops, but compared to the 90–120 PPI that’s been common on desktop displays since sometime in the late 1990s (>15 years!?), it’s substantially better. I highly recommend it, even at 30 Hz. (Especially recommended for anyone who spends more time looking at static content than animations.)
Also, for anyone on a Mac, definitely get the 24" 4k display, instead of the 28" or 32" version. (On Windows and Linux, I hear there are some issues with scaled apps, so YMMV.)
I can’t wait until all displays are at least this pixel density (300 ppi would be even nicer, but that’s probably another few years away), and we have large high resolution touchscreens to use drafting table style.