I've had some OS upgrades that went quite well and some that were complete disasters. But I've never had one where I downloaded the OS (legally), rebooted once and was done in under 15 minutes. Oh, and not to mention all my applications, documents, desktop wallpaper and other tweaks were all in place. Really seamless and worthy of imitation by anyone making an OS.
Another plus is that the Apple servers were up to the demand. And the OS seems to be working perfectly for the vast majority of users. Considering the complexity, I'm surprised we haven't heard about more problems.