Earlier quoted context omitted.
No, on the MBP (15" in my case; I would use a 17" if they still made that) I have to use scaling. Otherwise, as you imply, it's basically like having a super-sharp 12" screen (or something). I use it at the max That doesn't look quite as good, but it is still incomparably better than a low-res screen. But I mostly use a 27" 5K desktop monitor (a Dell UP2715K, which I believe uses the same panel as the high-res 27" iM…
yep. shame about them axing the 17 inch one. wow 5k have significant differences from 4k? maybe i should try that. but again, the forked dp cables means current (mobile at least) graphical power is still sorely lacking. and i'm still waiting for the 5k to be more available and being more affordable.
Intuitively, I expected 5K at 27" to be roughly the same as 4K at 24". But, nope. It's dramatically better than 4K at 24". I had them side by side in a 3-monitor setup for a time, but within about 30 seconds I though, ugh, I can't even stand to LOOK at these barbaric 4Ks ever again! and I unloaded them on craigslist.
But don't take my word for it; every Apple store has a 5K 27" iMac on display.
It doesn't actually look crisper to my eye than a "retina" MacBook Pro -- the huge difference, though, is that the 17" 5K panel is awesome at its native resolution. On the MBP you definitely need to use the "scaling" feature, which while much better than the sad old low-res generations of displays, nevertheless does add some fuzziness.
5K at 27" is the best thing readily available today, and I am not sure it needs to get much better than this (if at all) for the resolution of my eyeballs.
Spending $1500 for a computer monitor does kind of suck though, so yeah there's that, still... :-/