It's not a completely new OS in name or aesthetic, but the guts of Snow Leopard are radically different enough you may or may not be worried about upgrading right away.
There are minor issues with software compatibility, bugs and a virus or two. Some even reported that they lost their data during the upgrade process.
Well, how worried are you? Enough to delay the upgrade for a few weeks? Or are you going to do it the second you get home with the disc this weekend? Or somewhere in between?