Even though I have MS Office 2004 on the Mac, I still use NeoOffice/J when it's time to save a document I want to keep around. The documents are smaller (they're ZIP archives containing XML and any graphics you've used), and the document formats are openly specified and made available without any legal encumbrances or secret tricks.
If and when NeoOffice/J catches up with OpenOffice.org 2.0, they'll have another feather in their cap by supporting, as does OOo2, the OpenDocument OASIS Standard as its native file format. This format is based on OpenOffice.org's XML formats and guarantees that one's important documents are not locked into a proprietary box.
So by using NeoOffice/J or OpenOffice.org, you can future-proof your documents and be assured that you will always be able to use them regardless of any one company's product upgrade cycle.