With my Mac Mini I also got an Upgrade DVD. Now, I wasn’t too fussed about having Leopard on the Media Centre, but I did want it on my laptop. So, I thought, I’ll just insert it into the MacBook Pro, and install from there.
Of course, that would be good if it worked. Which it didn’t. According to the dialog box, I “Don’t have Mac OS X v.4 Installed”.
Which isn’t the case:

Using Disk Utility was odd - for some reason my internal drive was not being mounted by the system.
I’m now trying the disk on a G5 iMac. I don’t know if this is a deliberate inability to install as it is from a Mac Mini, or if it is just an odd error.
[later] And it appears like this DVD will not boot a G5 iMac. Or at least, not this one.
Wait a little bit. I’ve heard that leopard is the Vista of the Mac OSX… its unstable, lots of glitz, little function. :/
3 hours, 48 minutes after the fact.
I’m actually looking forward to some of the functionality - Spaces looks to be more supported than my current virtual desktop system, Desktop Manager. It will be nice to be able to actually force apps to run on particular desktops, instead of having to remember to switch to screen 4 before running WoW, for instance.
Time machine also looks good. I’m not real good at backing up, although with CarbonCopyCloner I’m getting better. More importantly this provides pretty darn good revision control.
Finally, Spotlight looks like it has been fixed at last. About time, since BeOS had amazingly quick queries, granted with smaller disks, but also with much slower hardware.
21 hours, 1 minute after the fact.