I'm pretty excited about this update! The Intel i9-9900K is a beast of a processor, and looking at Geekbench results for what are likely Hackintoshes seems to compare favorably to the 10-core chip in the iMac Pro on multicore, and is even faster with single core. I ordered the i9 with the Vega 48, 2TB SSD, and 8GB RAM which I am upgrading to 40GB total using matched pairs of 16x2 and 4x2 (included) for an extra $200 or so. It's going to be a great machine which will hopefully last me a long time (5-7 years). I've created a discussion thread over in the iMac forum for users to talk about which configuration they ordered and I go into an explanation of why I ordered what I did:
https://forums.macrumors.com/threads/what-2019-imac-did-you-order.2173991/.
Compared to an iMac Pro 10-core (again, comparing multicore scores mentioned above) with 32GB RAM (I'll have 40GB), 2TB SSD and Vega 56 I'll be saving nearly $2000 for a machine that has a similar (multicore) or maybe even slightly better (single core) CPU, more RAM, same SSD, and slightly slower GPU. Sure there are higher end options like 10Gbit ethernet, UHS-II SD card slot, and the darker colorway—but I don't think those along with a GPU that is maybe around 15% faster justifies the extra $2000 expense. And actually since it's a desktop why not get a 10Gbit dongle (which I won't need until my current 1Gbps speeds increase in my area) and I already have the Apple USB-C to UHS-II SD card adapter which I've used with my iPad Pro. Overall this is a great upgrade and I can't wait for it to get here!