I'd not say that it would be the quickest. The pokey pace of downloading is the biggest bottleneck. Perhaps its not fare to compare apple with fedora (or ubuntu) but when they update their OS, its takes forever for me to download. I usually try to do this at work as my companies internet connection is much faster but it takes hours nonetheless.
well if you account for the entire process of updating the OS, yes it probably is the quickest. If I need a CD, that requires going to the store, finding the product, waiting in line, driving home, then installing it; or waiting for it to be shipped to you. A disc or thumb drive won't magically appear at your house come upgrade time!
If its a download I could start it, go about my buisness doing whatever else I felt like until the download is complete then upgrade...
And in the future if I didn't want to wait for it to download again I could always throw it on a cheap 8gb thumb drive or a dual layer DVD. Then again if you do redownload it every time it would already come with all of the security patches and such and you wouldn't have to worry about downloading anything else after you're done with the install.
-edit-
beat me to it haha...
but yeah for those saying they prefer physical media, you know you could burn it to a dvd or copy it to a thumb drive once you have downloaded it right?