One of the cool things about FXG is that it works with Flex data-binding just as your would expect it to.
As you can see, there are still some bugs in the rendering, but that is to be expected as these are very early Flex 4 builds.
<?xml version="1.0" encoding="utf-8"?>
<mx:Application
xmlns:mx="library:adobe/flex/halo"
xmlns="http://ns.adobe.com/mxml/2009"
xmlns:gumbo="library:adobe/flex/gumbo"
verticalCenter="0" horizontalCenter="0"
layout="vertical">
<gumbo:Group verticalCenter="0" horizontalCenter="0">
<Rect width="300" height="200" radiusX="{radiusSlider.value}"
radiusY="{radiusSlider.value}">
<fill>
<SolidColor color="{fillColor.selectedColor}" />
</fill>
<stroke>
<SolidColorStroke weight="{strokeSlider.value}"
color="{strokeColor.selectedColor}" alpha="1" />
</stroke>
</Rect>
</gumbo:Group>
<mx:Label text="Corner Radius" />
<mx:HSlider minimum="0" maximum="100" id="radiusSlider" liveDragging="true"/>
<mx:Label text="Stroke Weight" />
<mx:HSlider minimum="0" maximum="20" value="5" id="strokeSlider" liveDragging="true"/>
<mx:Label text="Stroke Color" />
<mx:ColorPicker id="strokeColor" selectedColor="0x333333" />
<mx:Label text="Fill Color" />
<mx:ColorPicker id="fillColor" selectedColor="0x999999" />
</mx:Application>
You can of course, read more details on this in the FXG 1.0 Specification.
I also have a post on how to get started with Flex 4 FXG in Flex Builder 3.