For what it's worth, I did the default "upgrade" installation of Jaguar 10.2 on my PowerbookG4's 10.1.5/9.2.2 system that's pretty much as Apple sent it to me in 10.0.2, upgraded to 10.1 by CD and beyond by Software Update. (I did first prudently used Carbon Copy Cloner to back it all up to a firewire drive I can boot from in emergencies.) I reasoned I could always do one of the the more drastic options if that didn't work out. I have a pretty much vanilla system, with only a few things like Office v.X, Graphic Converter, Toast, Virtual PC, and ViaVoice installed on OS X (and a few older Classic apps like HyperCard, Painter and a uMax scanner application).
The whole process went flawlessly.
I had to change one Speech preference setting to get ViaVoice running, and rebuild one Office identity database on one account to assuage Entourage (it was probably already slightly munged.) Airport, DSL via Ethernet, dial-up, Office, browsers, mail, ViaVoice, Microsoft wireless intelimouse, DVD player, iTunes, Sherlock, Help, sleep, cover-closed operation docked to a larger monitor, and those Classic applications (including the scanner) all function fine (I'm not sure I'd say it was a big improvement, either; but I knew Quartz Extreeme didn't support my video going in.)
I think if you read the various forum chatter you discover that only people who have problems post, and those who have a good experience don't have much to say, so upgrades always sound scary.
I will now go knock on wood. images/icons/wink.gif