Here at my work, we use a mixture of Office 97 and OpenOffice.  The mix works
OK, but not as smoothly as our end-users would like.  The primary issue seems to
be the way documents are saved into native Office 97 format from OpenOffice. 
Many of our end-users call with issues or complaints of formatting or page setup
being lost in the transition back to Excel or Word.  My suggestion is the
development of a complex document with several features for both Word and Excel.
 The idea is that during QA, the documents are imported, then re-exported back.
 After re-exporting, the documents should be examined to determine how much of
the data, metadata, and formatting survived during the transition.

This may not seem like a large issue in an all-OOo environment, but in a mixed
setup (like ours) it can make the difference between adopotion of a new (and
superiour) technology, or a forced upgrade to Office XP.  For an example, please
corss-reference bug 9873, "Print Range Definition fails to export into Excel".