My current project involves entering a large number of pieces of data into a series of userforms, which populate fields and select certain content. I would like to hedge myself against the client asking at a later date for the ability to update a completed document (or to pull data from an older version into a newer version). To do this, I would like to store all of the user-entered values in the document.

Are there any down sides to using Document Variables vs CustomDocumentProperties? I have used Custom Properties in PowerPoint in the past, but have not used either in Word yet. I know that the custom properties are viewable by the user, so I would prefer to use variables, but are there any issues that I need to know about?

No not really. Except for the point you mentioned. Custom Properties are viewable, doc variables are not. At least not through the GUI.

Thanks. Doc variables it is.