I upgraded at launch on my Mac Pro with an OEM Apple RAID card.
The installer will update your firmware then install. After install it won’t boot.
Turns out they forgot to put the drivers in for the Apple OEM raid card. High Sierra cannot see any drives attached through the card.
Apple claims to support these older Mac Pro computers for this upgrade but can’t give me an eta for when they will add support back in.
If you have an older Mac Pro your system will be bricked by this upgrade.