Upgrading to Leopard while using mirrored monitor
Only upgrade if your not in the middle of a project. Sometimes things don't go as smooth as they should. Also, backup your main drive and settings to another drive before upgrading, so if you need to, you can downgrade to where you were, (just a word of caution, learned from experience.)
Personally, I have not upgraded yet, due to some third party software that doesn't play nice with Leopard yet and that I depend on daily. As far as I can see Leopard works quit well with it's own Apple software products.
You will also loose OS 9 classic mode under Leopard, it isn't supported in Leopard.
If your wondering, I did upgrade and then almost immediately downgraded after my third party software stop working.