Mathew Ingram notes that Apple is not immune to upgrade problems - the fanboys notwithstanding:
The growing number of reports about problems with Leopard, the new Mac OS, show that there is a lot more to it than that. I just heard from a friend -- a relatively recent convert to Apple PCs -- who said that the upgrade didn’t just present him with a blue screen (something that until now had been associated exclusively with Windows machines), but actually wiped out most of his data and a substantial number of applications as well. I don’t know whether his problems were a result of using the third-party Application Enhancer software or not, as some have reported.
I had decided to wait on Leopard, and it looks like it was a good idea. I'll give it a few months to sort out.