Import/Export works very well. Many customers use it. It's for converting a high quantity of documents. I believe that Rajal Shah/Juniper mentioned it in the talk he gave to the Arbortext PTC/User group last year.There aren't any limitations. Import/Export requires you to create a map that tells how to convert your Word markup into your DTD markup. Arbortext also provides the ability to do intermediate automated processing by giving you access to the conversion process (hooks) so you can do some clean-up or advanced post-processing before you get your final document. Does this make sense?
The cut/paste function works remarkably well.You can select all text in your document and then paste into Arbortext (assuming you paste where your DTD allows all the included content types) and it will paste and structure the entire thing. I've done this frequently when converting 1-off samples for prospective customers. If you're using a generic DocBook or DITA topic document, as flexible doctypes the conversion is pretty simple because these are well-known to Arbortext.
Here are my thoughts based on the experience of using Import/Export for converting MS Word -> S1000D.
The main limitation of Atx Import/Export (actually, of any similar solution) when talking about importing documents from MS Word is that the quality of end result is heavily depends on the quality (consistency) of input .doc files.
If input .doc/.docx documents are not consistent (e.g. have different formatting styles for the same data), you'd have to have several conversion templates and still do a lot of cleanup after converting.
But generally I agree with Liz - Import/Export works well and can be a great help.