Hi I have one of the new iMacs with 27" 5K w/Radeon Pro 580. I installed Windows 10 in Boot Camp - and had to make a few tweaks to get the keyboard mapping and the WiFi settings to work properly with 802.11ac, and stuff like that. But once Windows was installed and up-to-date, I installed the Oculus Rift drivers - and it all works fine, however, you do need to add a Thunderbolt-to-HDMI adapter as the Rift connects to HDMI. The Rift also needs 3 USB ports, so that's all good once you have that plugged-in. I purchased the Apple one, although there are alternatives. But the iMac doesn't have a HDMI port, hence the need for the Thunderbolt/USB-C adapter.
I ran the setup fine and it seemed to work perfectly. However, it continually warns that it doesn't meet the minimum requirements on spec. The compatibility tool from the Oculus website only points to the graphics card, which afaik should be on spec. Not sure what's going on there. I also tried Steam VR, but had a bit less success there - I encountered some random flickering on occasion. The big issue for me was the inability to set the focus correctly in the Rift - being short-sighted, I will have to figure out a way of fitting it over my glasses.
So on the whole it was fine - despite the warning about minimum specs, it seemed to work perfectly well. Good luck!