I've had the machine for a few hours now. I am about to do the Sierra update from El Capitan after my photos are done importing. Amazing machine! It's so refreshing to be excited about a computer instead of getting another Windows machine and knowing exactly what to expect.
I have my 24in Dell monitor hooked up as a second monitor and it's funny, before the iMac, I thought the monitor looked great, clear, etc... but now, side by side with the iMac screen, the iMac puts it to shame!!