I have ordered one, and hopefully it will be shipped in the next 4-5 days. But I took a trip out to the local best buy and they had the two 27" iMac's on display, and at casual glance, I was hard pressed to see any difference between the two screens. Now I will admit they didn't have any 4k video demo's running. I just pulled up Safari and it had the apple web site up on both. If you got close, you could see that the small text was cleaner on the 5K but you needed to be 4-5" from the screen to tell. From a seated position, I couldn't tell them apart.
Now, when I priced the cost difference between the two with an i7 and the fusion drive, the cost break wasn't that big that I went with the 5K as I did get a faster processor and on paper the better monitor. (but some say slower performing video)
I am wondering from those who have worked for a long time on the older 27" screen, if the new screen is better, easier on the eyes, or some how more noticeable than I found in the non perfect setup at Bestbuy.