Any time a new operating system comes out, one big worry is the compatibility of existing applications that you are already running. There's nothing worse than upgrading your OS and finding out that an application you rely on no longer runs. In time it all works out. But you don't want to be an early adopter if it keeps you from getting your work done.

With the release of the newest version of OS X, called Snow Leopard, this may be an issue since the OS is making lots of changes "below the covers". If you want to upgrade to Snow Leopard to get the performance improvements it's supposed to offer, but you don't want any of your apps to stop working, you can check the Snow Leopard compatibility matrix at: http://snowleopard.wikidot.com/