I have a 2017 5K 27" iMac and I'm very happy with it. I have room on my desk though so I also have a 25" monitor along side of it. The 25" isn't near as good a monitor as the iMac but.... for me, having a second monitor for PS is a boon. Handy for Skype as well.
I calibrate my 27" iMac with an x-rite i1 Display Pro. I've had no issues with that as long as I keep the x-rite software up to date.
I set the monitor up for D65, 100 luminance and standard 2.2 gamma. I'm happy with my prints. I print from LrC.
The 27" provides the ability to choose between several resolutions:
- 1600 x 900
- 2480 x 1152
- 2560 x 1440 (default)
- 2880 x 1620
- 3200 x 1800
I mostly use the default resolution because the 3200 x1800 turns text into an eye exam.
Hope this helps,