Great thread.
I am on a Mac Pro 5,1, about as maxed out as possible on CPU/Ram, and running High Sierra. I NEVER upgrade right away, either with OS X and/or iOS devices, I always wait for several iterations.
Recently started feeling some slow-down, "micro stalls", etc, so started researching upgrading to Mojave.
I realized that while I upgraded my CPU and RAM (2x3.46 6cores and 128gb of ram), I left my 10 year old Radeon 5770 in there, so I ordered a new Radeon 580 (8g) to replace this legacy GPU as preventative maintenance.
At the end of the day, I debating if I really want to update to Mojave given the stability of High Sierra, while dealing with some intermittent "circles of delay" that have popped up. The new GPU has metal support, so the door remains open.