Having only been used to rebuilding Windows machines, I was dreading it. I needn't have worried. A clean install of Snow Leopard took less than half an hour and, thanks to my recent experiment with trying MobileMe again, I was completely back up and running on a clean machine, with my old contacts, calendars, mail etc all intact in less than 40 minutes. Pretty impressive. It obviously took a while to download and install all of the updates, but it only needed one reboot and was happily applying the other updates in the background whilst I got on with other stuff.
Why aren't all machines built this way?!