Mac: How will your existing apps cope with Mountain Lion? In our tests the operating system has been very stable and compatible, but there will inevitably be some that cause trouble. If you want to double-check, the developers at Roaring Apps have compiled a compatibility database.
While the search function isn't perfect, you can browse the compatibility table by letter and immediately check compatbility. This table should help you determine if the upgrade is going to cause a serious issue with an important app you use every day.
App Compatibility Table [Roaring Apps]