Working on OpenXML? so, how easy it is to map the content controls in word document to custom XML?

You can use Content Control Toolkit, to assist you to create a data-bound document, and it's also a good developer tool for learning about the markup you need for binding controls to data.

Instead of typing the syntax one-by-one to edit/add/remove Custom XML parts, and manipulate content control properties, you can now use the Content Control Toolkit provides the ability to map content controls to custom XML nodes through a simple drag-and-drop UI. This really makes your life easier.

You can download the tool from Codeplex project, which developed by Microsoft's Matt Scott.
You can watch the video on video on Channel 9 how the tool works.

You may visit this Andrew's Blog, to see how to use the Content Control Toolkit.