Pretty nice, except some minor issues, read more here: https://forum.xda-developers.com/mate-9/development/roms-openkirin-aosp-collection-t3828804/how is omni on this phone?
I highly recommended it if you have an unlocked bootloader, works great with Magisk (only using Magisk for LKT though: https://forum.xda-developers.com/apps/magisk/xz-lxt-1-0-insane-battery-life-12h-sot-t3700688 ) and the camera is the best I've used (there's some lag on mode switch, but given that the Huawei camera app behaves strangely on custom ROMs this one is fine). Note that I use the included camera, not one from Magisk or elsewhere. Battery live is great (exceeding 6h SoT constantly), (since this is clean AOSP) my apps don't get killed at random, and they can now show scheduled notification and such (the main reason I've dropped EMUi in the first place).