I can't remember how I worked before I found opacity. It is simply genius! The possibility to use variables in the factories will give you a workflow you don't want to miss once you used it.
Even the latest version is from late 2009, it still works perfectly on ML, even the GUI may look a little bit outdated. I own many other tools with nice GUIs (e.g. Sketch) but those can't even be compared with this app. Once you understood how Opacity works, you can't live without it. I would call it "the essential tool for web, screen and UI designers".
Only one small thing I like to see improved: more factory locations (especially core FTP). Or release an API for other developers to write their own locations (using AppleScript, Shell script, ...).