Last night I upgraded my iMac to Leopard. Some notes about the process:

1. I spent the evening before the upgrade running SuperDuper to backup my entire hard drive to an external disk. Especially given this was the first OS upgrade I’d ever done of a Mac, I wanted to be extra-sure I had good recovery options. SuperDuper worked great, but the backup was extraordinarily slow – I got about 6MB/sec transfer speeds over a USB 2.0 connection. Amazingly, I could boot off the external USB drive just fine, although it was a lot slower loading.
2. I’m not quite sure what the purpose is of running the Leopard installer application on your existing Mac OSX installation, rather than just booting off the DVD. As best I can tell, it just sets your startup disk to your DVD drive and reboots. The only contrary evidence I have is that when I launched the installer from my external USB 2.0 drive, after the reboot the installer didn’t see my internal hard drive at all as an upgrade option. I rebooted a 2nd time, turned off the USB drive and used the Cmd-key to boot off the DVD again. This time it allowed me to select the internal drive.
3. I never saw an option in the installer to pick between an upgrade, archive install, or to wipe the drive and install clean. I wanted to try the upgrade first, so it didn’t particularly matter, but I still don’t know how to do a clean install if I wanted to, short of wiping the drive before running the installer.
4. In an amazing display of trust given it was my first OS X upgrade, I answered the initial questions and went away for an hour or two, had dinner, gave out candy to trick-or-treaters, etc. I have no clue how long the installation actually took.
5. When I finally did return to my iMac, I was stuck at the now somewhat-infamous Macintosh blue screen of death. Apple’s knowledge base article on the issue was all I needed to fix it, but it certainly wasn’t the most reassuring environment for a fix. Single user mode on Leopard is just like single-user mode in Linux: text based, read-only filesystems you need to fsck, etc. I was particularly amused by the instruction to “Restart normally” – how do you restart “normally” when there’s no OS X GUI? I know to type reboot, but not everyone does…
6. Aside from the BSOD, I had no issues upgrading. So far it feels snappy and has some very nice additions, but I’m still exploring, so I’ll leave it at that.