Wanted to report in on my personal experience with the iMac Pro.
My main use case is editing files off the Phase XF100 in Capture One. Previously I was using a lower-end iMac from about 3 years ago. That machine really struggled with the high-res files, with Capture One taking at least a few seconds between photos and sometimes 10-15 if you caught it wrong-footed while quickly browsing/zooming. Too bad, since the form factor, screen, etc. is great. The happy news is that the iMac Pro solves that issue nicely--flipping between photos is nearly instant and now I can't get it to delay for more that a second or three at any time. Importing and exporting photos doesn't seem to be very well threaded (watching activity monitor), so I doubt that the 10 core model that I got is substantially better that a modern top-spec iMac for this workload (Diglloyd has good numbers on this). For my workflows that doesn't matter anyway, as I mostly care about the interactive speeds.
A couple of interesting items for the tech-obsessed that I've observed:
1) Despite the tech specs saying that 4.5GHz is the top turbo boost speed, I've only been able to get it to 4.2Ghz (monitored via Intel Power Gadget) on single-threaded workloads. Hmm...
2) Apple says the last-level-cache on the 10-core machine is 23.75MB. That is substantially more than the normal Xeon-W (i.e. non-apple special model) and one of the nice hidden-bonus specs that should contribute to good single-core performance as well. The rub is that every system information program I've tried (including the built-in ones) reports 13.75 MB instead--the normal amount. Another hmm...
All in all, I think I probably would have been almost as happy with a loaded 5K iMac, but a few things pushed me over the edge for the iMac Pro, including the faster graphics card and having ECC ram, which I have a probably-irrational affinity for from past personal experiences. Finally, when you spec up that 5K iMac with 64GB of RAM and a 2TB SSD, it's running $5,350 anyway, so the premium isn't too insane.
Dave