I've been setting it up over the weekend and I have to say that I'm loving it. I've set it up from scratch, not using migration assistant so I've had my iMac running, working my way through the installed applications and re-installing them onto the new computer. I'm blown away by the speed of this thing and I've barely touched the surface of it.
What amazes me more than anything else is the power consumption. It's drawing about 15-16W while I'm using it now with all my normal stuff open, plenty of tabs in Safari etc. It's effectively just idling. To put that into perspective, a Raspberry Pi4 at idle draws about 4.5W.
I've seen plenty of talk on macrumours about coil whine but I don't have any. Yes, I can hear the fans running and it's fractionally louder than the iMac was at idle but the fans on the iMac used to ramp up at the slightest load - I've run a few things to give this a bit of a thrash but I've not heard the fans speed up at all.
All in all I'm very happy.