File format compatibility is probably the No. 1 reason IT departments would choose Office 2008 instead of the significantly cheaper iWork '08 or the entirely free OpenOffice 2.3. Compared to these non-Microsoft applications, Office 2008 proved itself significantly more fluent with Microsoft Office documents.
To test the file conversion fidelity of Office for Mac 2008, I used the same group of files I used when testing iWork's presentation, word-processing and spreadsheet applications in my review of iWork '08 last year (see links below) and found none of the formatting issued I'd encountered during those tests.
I also tested a set of OOXML documents from Office 2007 and found that, as with the older binary formatted documents I tested, the new OOXML files worked well with Office 2008. As with Office 2007, OOXML is now the default format for Office 2008, although I had the option of saving my documents in the older format. According to Microsoft, converters for enabling Office 2004 users to work with OOXML files will become available about eight weeks after Office 2008 hits retail shelves.