Mac OS X 10.8 introduced a caching system for app preferences ("cfprefsd"). While this probably increases performances for apps, it makes it harder for developers to manipulate preference values quickly for testing, because making changes directly to the plist files in the ~/Library/Preferences folder does not work any more with editors such as "Property List Editor.app" and the similar one in Xcode.
Therefore, I've just written a new tool that works like the plist editor in Xcode but uses the CFPreferences functions to modify the values instead of editing the plist files directly.
If you've worked with "Property List Editor" or Xcode, you should have no trouble using my tool. It supports even Copy + Paste the same way. The only things I have not implemented (yet) are Undo and changing the type of existing elements (the reason for the latter is that REALbasic's framework does not offer a way to show a popup menu inside a listbox cell, and so I was too lazy finding a work-around for this to imitate Xcode's UI).
It even has one feature that Xcode's editor doesn't: You can delete all entries at once with Select All, then delete.
Also note that due to the use of the CFPreferences API, any changes you make are immediately committed to the preferences as seen by other apps, just like when you use the "defaults" command. For the same reason, changes made to the prefs by other apps (or with the "defaults" tool) will be seen in this tool after a change, a front switch or by pressing the "Sync" button. Hence, there is no Save command - updates are instant. For that reason Undo would be quite useful. Alas, I can live without it for now.
The tool is free for your use. Get it here:
http://files.tempel.org/Various/OSX_Prefs_Editor/PrefsEditor.zip (Version 1.1.1)
It includes the REAL Studio (Xojo) source code as well. But I retain the copyright and distribution rights to the program, so please do not distribute the app yourself without getting my permission first!