It's really good!
A few minor bugs:
The tree view is cut off at the bottom of the left window (Win XP).
If you right click on a string gadget, every second click you can get a popup menu appear with 'undo, cut, copy, paste, etc.' and then you can change the text eg. 'string_0' that appears in the gadget.
If you resize a window as small as it will go (dragging from the bottom right hand corner), let go of the mouse, then enlarge it again, it has a completely black background (only when the grid is displayed).
If you select a gadget and then click inside a string gadget in the left hand window the wrong cursor appears until you move the mouse.
Selecting WindowCentered hides the gadgets in the window.
Selecting Window_0 doesn't change the properties gadget (but it works if you click the background of the window).
If a gadget is under or on top of another gadget then the corner resizing box isn't visible (but it can still be resized).
When resizing the window using the width and height boxes there is a minimum size but when resizing the window manually there isn't.