It is really nice to have the (blue) handles to move the top/left/bottom/right insets on a component in a GridBagLayout rather than typing the values into the constraints fields.
However, we have standard UI spacings we need to achieve. For example, 5 pixels between a control and the left/right edges of a dialog. Dragging the handles is nice, but you cannot tell until you let go what the inset value is going to be. It is impossible to tell the difference between 3, 4, 5, or 6 pixles from an edge. Only by checking the constraints value after dragging can you be sure you got "5" instead of "4" or "6".
My suggestion is that a tooltip window appear when you start dragging a handle that shows what the value will be if you let go (e.g. "left: 5"). As you drag this would continuously update. Likewise, the tooltips that appear when you hover over a handle should show the current value of that setting so it can be checked without opening the constraints and without starting a drag.
Just a thought...