Apart from the things below, 5.0.18 FOSS -> 6.0.1 FOSS on Ubuntu 8.04LTS went very smoothly!
1) SSH session quit in middle of upgrade (VPN dropped!!!) - scripts complete none-the-less
2) Zimlets un-deployed themselves - date & email still not deployable
3) Lost stats - a bit annoyed by this
Can't wait for 6.0.2...
End of day update:
I've spent most of the day showing people around the new features of GnR. The verdict: they LOVE it! Particular hits are fish-eye days in month calendar and composing/reading in new tabs. Personally, my favourite is the reading-pane-on-right - top marks ZCS team!