I installed 10.6 on 2 of my Macs, without any major problems. Some drivers need to be re-installed. I think if you need your Mac to make a living (like I do), and you want to try Snow Leopard, then it'll be smart to install on a seperate drive. Internal (for MacPro) or firewire/USB external. Like all brand new OSs, it has some bugs - till it's resolved.
Snow Leopard will get better - I like it already, and I don't have any issues opening CS4, MS Office, Toast, or any other apps. For people who need their Mac to make a living, my advice is ALWAYS try it out on another drive first. You can always clone it back anytime !
The update 10.6 disc will work on a brand new drive, with or without Leopard. I installed it on a Mac without 10.5 ! No problems.