I upgraded three machines of three different architectures from 9.3 to 10.0_RC5 today without much drama.
In all my years
Upgrading Amiga OS is fun but scary.
Upgrading Solaris is a lot of work.
Upgrading Windows is downright painful.
Upgrading Linux is severely aggravating.
Upgrading NetBSD is always a breeze.
The hardest part about upgrading NetBSD was always ordering the CD and waiting for it to arrive, and then getting the one machine with a CDROM drive to cooperate. But these days I can just download everything.
Today I bungled one upgrade by accidentally rebooting the remote terminal I was using. The only other hitch was my DHCP server wouldn't start (taking my whole network down) because I had an obsolete configuration option. Clearly spelled out in /war/log/messages, it took less than a minute to fix. Had this been Debian, I'd spend four days pulling my hair out, trying to fix fifty problems that are hard to find and hard to find solutions for.