Just to give a counter point, I read about people having problems with the Leopard => Snow Leopard upgrade so I was very cautious when it came time for me to up

Just to give a counter point, I read about people having problems with the Leopard => Snow Leopard upgrade so I was very cautious when it came time for me to upgrade. I upgraded from Leopard to Snow Leopard on my early 2009 Mac Pro and have not experienced any issues yet. I do however suspect that perhaps the reason I had such a smooth upgrade was because my Mac Pro was quite new and my installation of Leopard at the time it was upgraded to Snow Leopard was pretty much a pristine factory install, with very little if any cruft hanging around. Still these kinds of things make me happy that I keep an up to date bootable backup of my system drive and I'll be purchasing a separate external drive to use with Time Machine some time soon.