I've just installed Snow Leopard, Apple's new operating system version 10.6 and it's working very nicely on my quad Intel Mac Pro (Snow Leopard is the first version of OS X that does not support older Power PC Macs). As it is optimised for Intel the speed benefits are apparent as soon as you start using it and it has a number of attractive new features.

There are a few things to watch, however.

I have client reports of simply putting the new Snow Leopard disk in and upgrading. It didn't go well. These Macs have, in some cases, become extremely unstable.

There are further online reports of Snow Leopard blocking users from installing any more software after an upgrade. The installer appears to stop working. I highly recommend backing up your drive, erasing it and doing a fresh install of Snow Leopard before restoring your files and preferably doing a fresh install of your programs.

Printers: If you are using an older printer that relies on AppleTalk you won't be able to use it from your new Snow Leopard installation because AppleTalk is no longer supported. If you can still access from another Mac running an older OS on your network you can turn Printer Sharing on and use it that way.
There are a few programs that will now work on Snow Leopard (all of mine worked). There is a list here of programs that will not work on Snow Leopard here.