After a seamless upgrade everything working fine – except the dropped support for a few well used tools.
VMware fusion (V3.1.4 or V4) – Not supported
Mail has dropped RSS support, shame as that was really useful – try Vienna its free and looks like it could be a good tool Website
To install however you will need to remove the new security preference that stops you using apps from out side the appstore. Goto Preferences > Security & Privacy > Allow Applications downloaded from : (change to) Anywhere
Apple’s own Mountain Lion support overview and discussion boards are a good place to check for issues before upgrading.
Roaring Apps have been working hard on a compatibility list – look here first before upgrading!!!!